Add now required .readthedocs.yaml

[FrostyX]

Fix #2771

[FrostyX]

I temporarily reconfigured https://readthedocs.org/projects/python-copr/ to build from this branch instead of main. It seems to work fine, I even tried to change some text in the documentation and it appeared on the rebuilt page.

I didn't test the other documentations because it is a bit tedious to configure (this branch didn't appear in the readthedocs project settings, and I needed to workaround it by readthedocs/readthedocs.org#9367 (comment)). But expect them to work as well.