Add aria-label to docs search field

Fixes #21560. Pretty sure this wasn't intentional, so should be fine to remove.

Fixes #21699

* rename order utilities to intended class names

* Documentation fixes.

The actual available size of the navbar toggler icon is 30px, but the
SVG uses a 32 unit grid. This commit uses a 30 unit grid and updates
icon accordingly.

- Correctly places the _custom.scss file first in the other two buids.
- Also removes the clearfix mixin from the grid build because flexbox yo.

* Remove most of custom cursor: pointer; implementations

* turns out that's not so much of a thing

The actual navbar height is 3.5rem.

Header will now display in the Contents section.

Update Mix and match section with correct responsive classes
for stacking cols.

* Get this party started by removing mention of Normalize.css

* Nuke the old comment, consolidate to a single line and number as appropriate

* Bring over styles for HTML element from Normalize to Reboot

* Move margin override for body element from Normalize to Reboot

* Drop the block reset for HTML5 elements in IE9- from Normalize given we dropped IE9 support

* Building on previous commit, do the same thing for figure, figcaption, and main

* Remove IE9- display from Normalize given our browser support

* Drop IE8 figure margin because we're IE10+

* No need for the h1 overrides because we reset these font and margin styles anyway in _type.scss already

* Drop Safari 6 b and strong normalization because we're Safari 8+

* Remove mark styles for IE9- from Normalize

* Remove old iOS audio fixes from Normalize

* Remove IE9- display for progress from Normalize

* Remove more IE9- rules from Normalize

* One more IE9- dis...

* Make container in navbar to occupy full width even on smallest breakpoint

* Fix typo in comment

* Apply fix only on smaller breakpoint

* drop the comment as chrome doesn't have a bug here

* remove obvi comments

* proper fix for #21801

- ensures .progress-bar text is vertically centered should the height of the .progress-bar change by matching the line-height and height values
- we can't use flexbox here because the inner text of an element doesn't count as a flex item

* fix segmented buttons

changed flex-direction to row
centering is now done by align-items instead of justify-content

this way there is no need for flex property on the .btn inside .btn-group

* Update _input-group.scss

Add missing closing tag in scrollspy visual test

- It is unclear that the use of the card component is necessary
when extending the collapse behavior to behave like an accordion.

On small screens (< 576px), all documentation pages containing
`.bd-example` styled tags, show the horizontal scrollbar because of a
wrong horizontal margin property.

This PR sets the horizontal margin on gutter width basis instead of rem.

This is semantically correct and works just as well.

* Remove unused scss variables

* Remove unnecessary space

* Remove $zindex-navbar from docs

* Re-add $progress-box-shadow

* Add alert divider (<hr />)example in docs

* <hr/> => <hr> per @vsn4ik suggestion