Make ci-templates more official for fd.o?

So ci optimization and common standards have become a bit mission critical for fd.o. I think it'd be good to have one common home for shared optimizations and howtos. Stuff like making sure artifacts are small, network bw isn't wasted, caching is done the same way (since we have multiple runner farms). But also docs for CI farms for how tow to set up a caching server shared among all runners to avoid needless downloads from gitlab.fd.o

This here seems like a good home for all this, and we could make it official by moving the project to the freedesktop.org/ space.

Thoughts?

@daenzer @ndufresne @anholt @daniels @__tim @bentiss