* 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

- Creates new flexbox grid Sass file in our docs assets
- Updates the Gruntfile to compile said new Sass file and minify the output
- Update notice on flexbox docs page for how it works
- Only enable compiled flexbox grid CSS in hosted docs site, not in dev (for easier and specific debugging of all flexbox features)

Ports #18872 to v4
[skip sauce]

For easier debugging.
Just like we already do for the JS.

[skip sauce]

Fixes potential problems with iOS9's new viewport behavior - see for
instance https://forums.developer.apple.com/thread/13510

Move them to footer, and remove duplicate ones.

[ci skip]

[ci skip]

Related to #17314.

[ci skip]

- 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

Anchor.js 1.0.0 drops the need to use anchor.css, so we remove it
from the codebase and move any leftover styles over to docs.css.

Closes #16488 by merging it.

Refs:
https://github.com/h5bp/html5-boilerplate/commit/1c4fcf32142e888946db579df0efbb52542e0ed1
https://github.com/h5bp/html5-boilerplate/commit/345925f04384b59929d8d39ba428cef548921408

[ci skip]

[ci skip]

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

This way, it works in all browsers.

The validator doesn't choke anymore on this and even though it's not spec compliant, it's needed for some IE versions. So until we move favicon.ico in root, this should do the job.

Closes #13379.

X-Refs:
https://github.com/twbs/bootstrap/commit/b2a67ffe55f5fedb0ae1d5dff331687c3e744be0
https://github.com/twbs/bootstrap/commit/c97e6d5cad74d20257d28734f85311ef5d00b560

The Google Analytics account we use cannot currently be upgraded to Universal Analytics. In order to keep using Analytics, we have to stick with Classic for now.