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
add iOS and versions to Safari percent rounding warning; fixes #10723
Better documentation and optimized examples for grid classes
Update Gruntfile.js fixing undefined jQuery check

The check to see if jQuery was defined as incorrect causing the "Bootstrap requires jQuery" message to only be displayed if jQuery is defined and has a value which equates to `false`.
Added abridged form of 'What's included' docs to README
Merge pull request #10863 from DeanMarkTaylor/patch-2

Update Gruntfile.js fixing undefined jQuery check
rm unnecessary backslashes from jqueryCheck
Moved 'getting started' link out of 'What's included' section in README
Apply help-block class to help text in customize.html
Update qunit dependency
Merge pull request #10869 from ZDroid/patch-2

Update qunit dependency
Remove .ruby-version due to https://github.com/twbs/bootstrap/commit/4f20a29c329aa58919919426cbd2190fabfdb723
Remove "Remove margin on the body" from base css docs

Normalize did that.