Docs: replace CircleCI build with ReadTheDocs (#5279)

The documentation was being built on CircleCI instead of GHA like the
rest of the CI pipeline, because the former would allow the built
HTML documentation to be inspected as an artifact, whereas for the
latter this wasn't possible.

It is now possible to do the same with ReadTheDocs, and since we already
use that to host the documentation, that means we can now drop CircleCI.

Cherry-pick: 7fad822