Add a general getting started guide.

The general getting started guide provides a single entry-point
into the installfest guides and properly pushes people into the
correct guide version for their Rails installation.

Updates the front-page link to head to the general getting started
guide instead of the Rails 3.2 guide.
I'd love to have like a sidenote section of the guides which would allow us to annotate certain areas with additional information and/or links to help people through. I'm not sure how to integrate that in with markdown though.