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.
Merge pull request #32 from reinteractive-open/feature/rails_version_detection

Add a general getting started guide.