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 clone protocol from git:// to https://

Cloning with git: protocol doesn't allow to change anything in `.git/`, like when using `git pull`.
Correct grammar

Jekyll is the third person singular (it), so it should be 'its documentation', not 'their documentation'.
Change border to border-radius

Border isn't set, but border-radius is.
Merge pull request #10932 from ZDroid/patch-1

Correct grammar
Jumbotron: remove the hard-coded font-size from jumbotron.less and moved it into variables.less where other .font-size-xxxxx things now live.
Fix #10948: Clarify navbar example target in docs
merge in new ads
Change all navbar example targets

According to comment on https://github.com/twbs/bootstrap/issues/10948#issuecomment-25751889 .
Merge pull request #10950 from twbs/docs-navbar-class

Fix #10948: Clarify navbar example target in docs
Insert comma to fix validation warning

BS doesn't support IE6/IE7, so comma can be inserted.
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The Jumbotron font-size LESS variable now conforms to the '@{bootstrapClass}-font-size' naming scheme.