Update the ci docs with the new process and setup.

This is a pure docs update with the goal of helping other devs be able to walk into the CI system and find their way around. It provides some debugging tips and descriptions of the moving parts.

I've left the older instructions and noted them as bitrotten per the old Hacking docs. Those notes deal with starting up and dealing with a live Juju environment which CI does not use. Those notes might prove useful in bringing that back up at some point in the future.