Add documentation explaining new build system for contributors to understand better.

[safern]

Since the recent move we need docs explaining how new builds work, what is available to trigger, how to do so, how to diagnose a build failure, how to find test results, etc.

[Wraith2]

Accidental closure?

[safern]

LOL, I mentioned it in my PR, saying I still needed to fix this issue and GH closed it when I merged.