Add Welcome section to docs

* Fixes unrelated typo in readme
* Add new Welcome section with history, team, callouts, and community
subsections
* Redesigns callouts to be a bit lighterweight with colored headings
shorter avatar urls
use irc link
Merge branch 'master' into docs_welcome
Copy edits from @cvrebert and @juthilo
dat @juthilo edit
dat hash
move to separate about page
Change border to border-radius

Border isn't set, but border-radius is.
Jumbotron: remove the hard-coded font-size from jumbotron.less and moved it into variables.less where other .font-size-xxxxx things now live.
Insert comma to fix validation warning

BS doesn't support IE6/IE7, so comma can be inserted.
The Jumbotron font-size LESS variable now conforms to the '@{bootstrapClass}-font-size' naming scheme.
The customizer now displays and collects the new Jumbotron font-size LESS variable
Good catch by @juthilo - omitted the modified  in a prior commit.
address #10965; mention Glyphicons-over-file:// problem in Carousel example
use https:// for CDN links

- works when viewing page via file://
- sets a good example security-wise
fix typo in Getting Started docs prose
Merge pull request #11018 from twbs/use-https

use https:// for CDN links
Nicer docs license name

DOCUMENTATION-LICENSE is so long.
documentation that multiple modals are not supported