docs: Upgrade to Sphinx 3

Still needs an update of the CI container before it can be merged. Hawkmoth support for Sphinx 3 is working well, the only bigger issue was a problem in Sphinx itself, see https://github.com/sphinx-doc/sphinx/issues/8945 and commit f9f33dc9 ("hack(docs): Add workaround for Sphinx issue #8945").

@q3k, I updated the Dockerfile, can you rebuild and push that to your registry please?