* 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

Replace the leftover instances with .m-x-auto (and .d-block when necessary)
Closes #19665
Refs #19102
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Fix #17166

Related to #17314.

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- 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

remove tabindex=-1 as it was causing a weird select state in latest safari on yosemite when clicking anywhere in the negative space of the header

Add tabindex="-1" to main #content to work around bug in Chrome (and old
WebKits) and IE
Update advice in accessibility section
Include patch in docs CSS to suppress outline when #content is focused
(particularly as Chrome sets focus as result of mouse click for
tabindex="-1" elements)

Docs: Refactor to use .less file instead, importing Bootstrap vars and mixins, and cleaning all the things up

...at least if you're a writing nerd :P "Bootstrap ships with vanilla CSS, but *its* source code..."