A long overdue rewrite of the accessibility section - instead of the few
snippets of strangely superficial and out-of-context advice (skip links,
use correct heading levels), this tries to answer some of the
fundamental questions about "is Bootstrap accessible", with emphasis on
the fact that the final result will depend in large part on what BS is
applied to/on (since BS relies on the markup etc authored by
developers). This also sets out our ambition to have things work for
keyboard and assistive tech users, and that we strive to make all our
examples etc accessible and semantic.

* Changes based on @mdo's feedback

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=454172#c22

[skip sauce]

Closes #20165

[skip sauce]

https://crbug.com/262171 was closed as a duplicate of https://crbug.com/454172

[skip sauce]

- 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

- Replaces manual use of .bd-callout with {% callout [type] %}
- Rearranged some callouts for proximity to others
- Turned long lists of callouts--like those on tooltips, plugings, etc--into a list because holy shit that's overwhelming