move documentation off readthedocs

[prandla]

readthedocs shows ads on our docs now. we already have cms-dev.github.io, we could serve docs from there.

note for generating docs ourselves: dev-dependencies.txt pins an ancient version of sphinx. readthedocs actually completely ignores this and builds with latest sphinx (and our docs use a sphinx 8.x feature in at least one place). we can't just bump our declared sphinx dependency because that'll pull in a newer version of jinja (which is pinned to 2.x because of the tornado problem). so for now i think we should just remove sphinx from our dev-dependencies and install the right version in some separate build script.

[wil93]

It would make sense. Probably not a high priority (the ads are not very disruptive)