clean up front page's contributors section

http://docs.opendaylight.org/en/latest/index.html

what do we really want to list there? Is that for contributors in general or for people that
specifically want to contribute to projects. currently we have docs to help with gerrit
setup, how to use our infra, etc. But, we also have a few links to project specific (e.g.
netvirt, sfc) docs.

I think it should just be higher level broad docs for new contributors to take advantage.