Redo the getting started section to mention BootstrapCDN

Alternative solution to #7701. Instead of only including links in a template, I'd like to call out BootstrapCDN as a proper "getting started" alternative. I've included dummy URLs for now given BS3 isn't out yet (obviously) and some basic text around using CDN links instead of local copies.

A few questions/concerns:

* With v3, this will cease to be "Twitter Bootstrap" for good—it'll just be Bootstrap. Can we drop the "twitter-" part of the URLs with this?
* What about linking to just the absolute latest releases? Perhaps we don't want to do this, but I figured I'd throw it in there to see what folks think.

/cc @jdorfman