Docs: accessibility and IE fixes for SVG-based top navigation (#22848)

* Add explicit aria-label attributes to <svg> based links

As some browser/AT combinations don't seem to fully support grabbing the
`<title>` from inside the `<svg>` as the link text/name to announce
(e.g. Firefox+NVDA)

* Add <title> to Bootstrap logo <svg>

* Add <title> to menu <svg>

* Add explicit focusable="false" to <svg> icons

Due to a bug/"feature" in IE, `<svg>` elements are usually separately
focusable, which then results in links with `<svg>` icons being
essentially focused twice (once for the link, and once for the `<svg>`
inside, which is then also non-interactive)

_includes/docs-navbar.html

 <header class="navbar navbar-inverse navbar-expand-md flex-column flex-md-row bd-navbar">
-  <a class="navbar-brand mr-0 mr-md-2" href="{{ site.baseurl }}/">
+  <a class="navbar-brand mr-0 mr-md-2" href="{{ site.baseurl }}/" aria-label="Bootstrap">
     {% include icons/bootstrap.svg width="36" height="36" class="d-block" %}
   </a>
 
@@ -43,17 +43,17 @@
     </li>
 
     <li class="nav-item">
-      <a class="nav-link p-2" href="{{ site.repo }}" target="_blank">
+      <a class="nav-link p-2" href="{{ site.repo }}" target="_blank" aria-label="GitHub">
         {% include icons/github.svg class="navbar-nav-svg" %}
       </a>
     </li>
     <li class="nav-item">
-      <a class="nav-link p-2" href="https://twitter.com/{{ site.twitter }}" target="_blank">
+      <a class="nav-link p-2" href="https://twitter.com/{{ site.twitter }}" target="_blank" aria-label="Twitter">
         {% include icons/twitter.svg class="navbar-nav-svg" %}
       </a>
     </li>
     <li class="nav-item">
-      <a class="nav-link p-2" href="{{ site.slack }}" target="_blank">
+      <a class="nav-link p-2" href="{{ site.slack }}" target="_blank" aria-label="Slack">
         {% include icons/slack.svg class="navbar-nav-svg" %}
       </a>
     </li>

_includes/icons/bootstrap.svg

-<svg {% if include.class %} class="{{ include.class }}"{% endif %}{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %} viewbox="0 0 612 612" xmlns="http://www.w3.org/2000/svg"><path fill="currentColor" d="M510 8a94.3 94.3 0 0 1 94 94v408a94.3 94.3 0 0 1-94 94H102a94.3 94.3 0 0 1-94-94V102a94.3 94.3 0 0 1 94-94h408m0-8H102C45.9 0 0 45.9 0 102v408c0 56.1 45.9 102 102 102h408c56.1 0 102-45.9 102-102V102C612 45.9 566.1 0 510 0z"/><path fill="currentColor" d="M196.77 471.5V154.43h124.15c54.27 0 91 31.64 91 79.1 0 33-24.17 63.72-54.71 69.21v1.76c43.07 5.49 70.75 35.82 70.75 78 0 55.81-40 89-107.45 89zm39.55-180.4h63.28c46.8 0 72.29-18.68 72.29-53 0-31.42-21.53-48.78-60-48.78h-75.57zm78.22 145.46c47.68 0 72.73-19.34 72.73-56s-25.93-55.37-76.46-55.37h-74.49v111.4z"/></svg>
+<svg {% if include.class %} class="{{ include.class }}"{% endif %}{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %} viewbox="0 0 612 612" xmlns="http://www.w3.org/2000/svg" focusable="false"><title>Bootstrap</title><path fill="currentColor" d="M510 8a94.3 94.3 0 0 1 94 94v408a94.3 94.3 0 0 1-94 94H102a94.3 94.3 0 0 1-94-94V102a94.3 94.3 0 0 1 94-94h408m0-8H102C45.9 0 0 45.9 0 102v408c0 56.1 45.9 102 102 102h408c56.1 0 102-45.9 102-102V102C612 45.9 566.1 0 510 0z"/><path fill="currentColor" d="M196.77 471.5V154.43h124.15c54.27 0 91 31.64 91 79.1 0 33-24.17 63.72-54.71 69.21v1.76c43.07 5.49 70.75 35.82 70.75 78 0 55.81-40 89-107.45 89zm39.55-180.4h63.28c46.8 0 72.29-18.68 72.29-53 0-31.42-21.53-48.78-60-48.78h-75.57zm78.22 145.46c47.68 0 72.73-19.34 72.73-56s-25.93-55.37-76.46-55.37h-74.49v111.4z"/></svg>

_includes/icons/github.svg

-<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 512 499.36"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %}><title>GitHub</title><path d="M256 0C114.64 0 0 114.61 0 256c0 113.09 73.34 209 175.08 242.9 12.8 2.35 17.47-5.56 17.47-12.34 0-6.08-.22-22.18-.35-43.54-71.2 15.49-86.2-34.34-86.2-34.34-11.64-29.57-28.42-37.45-28.42-37.45-23.27-15.84 1.73-15.55 1.73-15.55 25.69 1.81 39.21 26.38 39.21 26.38 22.84 39.12 59.92 27.82 74.5 21.27 2.33-16.54 8.94-27.82 16.25-34.22-56.84-6.43-116.6-28.43-116.6-126.49 0-27.95 10-50.8 26.35-68.69-2.63-6.48-11.42-32.5 2.51-67.75 0 0 21.49-6.88 70.4 26.24a242.65 242.65 0 0 1 128.18 0c48.87-33.13 70.33-26.24 70.33-26.24 14 35.25 5.18 61.27 2.55 67.75 16.41 17.9 26.31 40.75 26.31 68.69 0 98.35-59.85 120-116.88 126.32 9.19 7.9 17.38 23.53 17.38 47.41 0 34.22-.31 61.83-.31 70.23 0 6.85 4.61 14.81 17.6 12.31C438.72 464.97 512 369.08 512 256.02 512 114.62 397.37 0 256 0z" fill="currentColor" fill-rule="evenodd"/></svg>
+<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 512 499.36"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %} focusable="false"><title>GitHub</title><path d="M256 0C114.64 0 0 114.61 0 256c0 113.09 73.34 209 175.08 242.9 12.8 2.35 17.47-5.56 17.47-12.34 0-6.08-.22-22.18-.35-43.54-71.2 15.49-86.2-34.34-86.2-34.34-11.64-29.57-28.42-37.45-28.42-37.45-23.27-15.84 1.73-15.55 1.73-15.55 25.69 1.81 39.21 26.38 39.21 26.38 22.84 39.12 59.92 27.82 74.5 21.27 2.33-16.54 8.94-27.82 16.25-34.22-56.84-6.43-116.6-28.43-116.6-126.49 0-27.95 10-50.8 26.35-68.69-2.63-6.48-11.42-32.5 2.51-67.75 0 0 21.49-6.88 70.4 26.24a242.65 242.65 0 0 1 128.18 0c48.87-33.13 70.33-26.24 70.33-26.24 14 35.25 5.18 61.27 2.55 67.75 16.41 17.9 26.31 40.75 26.31 68.69 0 98.35-59.85 120-116.88 126.32 9.19 7.9 17.38 23.53 17.38 47.41 0 34.22-.31 61.83-.31 70.23 0 6.85 4.61 14.81 17.6 12.31C438.72 464.97 512 369.08 512 256.02 512 114.62 397.37 0 256 0z" fill="currentColor" fill-rule="evenodd"/></svg>

_includes/icons/menu.svg

-<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 30 30"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %}><path stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-miterlimit="10" d="M4 7h22M4 15h22M4 23h22"/></svg>
+<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 30 30"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %} focusable="false"><title>Menu</title><path stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-miterlimit="10" d="M4 7h22M4 15h22M4 23h22"/></svg>

_includes/icons/slack.svg

-<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 512 512"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %}><title>Slack</title><path fill="currentColor" d="M210.787 234.832l68.31-22.883 22.1 65.977-68.309 22.882z"/><path d="M490.54 185.6C437.7 9.59 361.6-31.34 185.6 21.46S-31.3 150.4 21.46 326.4 150.4 543.3 326.4 490.54 543.34 361.6 490.54 185.6zM401.7 299.8l-33.15 11.05 11.46 34.38c4.5 13.92-2.87 29.06-16.78 33.56-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18l-11.46-34.38-68.36 22.92 11.46 34.38c4.5 13.92-2.87 29.06-16.78 33.56-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18l-11.46-34.43-33.15 11.05c-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18c-4.5-13.92 2.87-29.06 16.78-33.56l33.12-11.03-22.1-65.9-33.15 11.05c-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18c-4.48-13.93 2.89-29.07 16.81-33.58l33.15-11.05-11.46-34.38c-4.5-13.92 2.87-29.06 16.78-33.56s29.06 2.87 33.56 16.78l11.46 34.38 68.36-22.92-11.46-34.38c-4.5-13.92 2.87-29.06 16.78-33.56s29.06 2.87 33.56 16.78l11.47 34.42 33.15-11.05c13.92-4.5 29.06 2.87 33.56 16.78s-2.87 29.06-16.78 33.56L329.7 194.6l22.1 65.9 33.15-11.05c13.92-4.5 29.06 2.87 33.56 16.78s-2.88 29.07-16.81 33.57z" fill="currentColor"/></svg>
+<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 512 512"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %} focusable="false"><title>Slack</title><path fill="currentColor" d="M210.787 234.832l68.31-22.883 22.1 65.977-68.309 22.882z"/><path d="M490.54 185.6C437.7 9.59 361.6-31.34 185.6 21.46S-31.3 150.4 21.46 326.4 150.4 543.3 326.4 490.54 543.34 361.6 490.54 185.6zM401.7 299.8l-33.15 11.05 11.46 34.38c4.5 13.92-2.87 29.06-16.78 33.56-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18l-11.46-34.38-68.36 22.92 11.46 34.38c4.5 13.92-2.87 29.06-16.78 33.56-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18l-11.46-34.43-33.15 11.05c-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18c-4.5-13.92 2.87-29.06 16.78-33.56l33.12-11.03-22.1-65.9-33.15 11.05c-2.87.82-6.14 1.64-9 1.23a27.32 27.32 0 0 1-24.56-18c-4.48-13.93 2.89-29.07 16.81-33.58l33.15-11.05-11.46-34.38c-4.5-13.92 2.87-29.06 16.78-33.56s29.06 2.87 33.56 16.78l11.46 34.38 68.36-22.92-11.46-34.38c-4.5-13.92 2.87-29.06 16.78-33.56s29.06 2.87 33.56 16.78l11.47 34.42 33.15-11.05c13.92-4.5 29.06 2.87 33.56 16.78s-2.87 29.06-16.78 33.56L329.7 194.6l22.1 65.9 33.15-11.05c13.92-4.5 29.06 2.87 33.56 16.78s-2.88 29.07-16.81 33.57z" fill="currentColor"/></svg>

_includes/icons/twitter.svg

-<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 512 416.32"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %}><title>Twitter</title><path d="M160.83 416.32c193.2 0 298.92-160.22 298.92-298.92 0-4.51 0-9-.2-13.52A214 214 0 0 0 512 49.38a212.93 212.93 0 0 1-60.44 16.6 105.7 105.7 0 0 0 46.3-58.19 209 209 0 0 1-66.79 25.37 105.09 105.09 0 0 0-181.73 71.91 116.12 116.12 0 0 0 2.66 24c-87.28-4.3-164.73-46.3-216.56-109.82A105.48 105.48 0 0 0 68 159.6a106.27 106.27 0 0 1-47.53-13.11v1.43a105.28 105.28 0 0 0 84.21 103.06 105.67 105.67 0 0 1-47.33 1.84 105.06 105.06 0 0 0 98.14 72.94A210.72 210.72 0 0 1 25 370.84a202.17 202.17 0 0 1-25-1.43 298.85 298.85 0 0 0 160.83 46.92" fill="currentColor"/></svg>
+<svg{% if include.class %} class="{{ include.class }}"{% endif %} xmlns="http://www.w3.org/2000/svg" viewbox="0 0 512 416.32"{% if include.width %} width="{{ include.width }}"{% endif %}{% if include.height %} height="{{ include.height }}"{% endif %} focusable="false"><title>Twitter</title><path d="M160.83 416.32c193.2 0 298.92-160.22 298.92-298.92 0-4.51 0-9-.2-13.52A214 214 0 0 0 512 49.38a212.93 212.93 0 0 1-60.44 16.6 105.7 105.7 0 0 0 46.3-58.19 209 209 0 0 1-66.79 25.37 105.09 105.09 0 0 0-181.73 71.91 116.12 116.12 0 0 0 2.66 24c-87.28-4.3-164.73-46.3-216.56-109.82A105.48 105.48 0 0 0 68 159.6a106.27 106.27 0 0 1-47.53-13.11v1.43a105.28 105.28 0 0 0 84.21 103.06 105.67 105.67 0 0 1-47.33 1.84 105.06 105.06 0 0 0 98.14 72.94A210.72 210.72 0 0 1 25 370.84a202.17 202.17 0 0 1-25-1.43 298.85 298.85 0 0 0 160.83 46.92" fill="currentColor"/></svg>