Refactor project infrastructure docs.

The goal is to re-orient the documentation as an introduction for new
contributors and a reference for all contributors.

Change-Id: I8702a5ace908c7618a6451bbfef7fc79b07429ff
Reviewed-on: https://review.openstack.org/30515
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins