* Fixed dead link to browser list

The link to the list of browsers supported through Autoprefixer was dead, now updated to a working link.

* Updated label to display new file path

Changed the labeling on the updated link to show read as the new updated file path.

The link to package.json in the v4 docs was linking to package.json in the v3 source.  Fixed to point to the correct v4 location.  A change like this will need to be made for all branches on version 4 if you want the docs to be linked correctly (github may be smart enough to follow the link to the current branch if you are on github but it was wrong from the main site).

* 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 #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

[skip sauce]

Closes https://github.com/twbs/bootstrap/issues/17493

- 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

move compilting to build-tools, rewrite some grunt stuff, add local docs with jekyll section based on readme's more detailed walkthrough