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
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
Nicer docs license name

DOCUMENTATION-LICENSE is so long.
Combine separate efforts for grid mixins into one, cleanup and reorganize
restore var to customizer
Sort dependencies A-Z
Document caret /cc #10102
remove screen from all docs media queries