Closes #17319 by merging it.

.visible-xs(-block) => .hidden-sm-up

Fixes #17263.
[skip sauce]

update broken holder.min.js link

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

Shouldn't really happen, but China for example has blocked Google so this should help.

Closes #16599 by merging a tweaked version of it.

as role="menu" is a very specific (and strict) ARIA pattern for
desktop-like application menus, and our dropdowns are often used
as pure navigation dropdowns, this change abandons ARIA menus for
a more open-ended and light-weight approach
(see http://heydonworks.com/practical_aria_examples/#submenus and
http://www.w3.org/WAI/tutorials/menus/flyout/#improve-screen-reader-support-using-wai-aria)
note that in dropdown.js, switched to now target ``.dropdown-menu``
instead of ``role["menu"]`` - this also prevents bootstrap scripts
from "bleeding" into non-bootstrap components on the same page.
also removed the ``role=["listbox"]`` part, which appears to be
vestigial/unused (only place in bootstrap that uses that
role are carousels, and their key handling is done separately)

http://www.w3.org/TR/wai-aria/roles#separator

[ci skip]

Closes #15291 by merging it.

Document in the source code that the justified nav is meant for single line list items only. This should help prevent Issues like #15906 in the future.

And add explanatory comments to the Carousel & Dashboard examples regarding holder.js

[skip sauce]

The revert was incomplete.

Fix issue #15601 — Position of X-UA-Compatible, charset and viewport

Add comment to all examples

The non-responsive is correctly saying 2 instead of 3

Fix typo on Utilize

When the screen is shrunk below the md threshold, all the featurette columns stack.
However, since the second featurette is "backwards", when stacked the second featurette
looked awkward as the image came before the heading.

In order to see the changes, you must shrink the browser to below the md threshold.

Use a simple blank.gif and change the background color instead of duplicating stuff.