Add initial documentation on testing

My end goal here is to have documentation on how to make the most of openQA testing.

As part of that, we need to document GNOME OS. As part of that, we need to document the gnome-build-meta integration repo.

This draft is mainly an attempt to get the overall structure of the documentation in place, so please prioritize that when reviewing.

https://gitlab.gnome.org/Teams/documentation/developer-www/-/issues/34