Use reStructuredText and Sphinx in order to generate the documentation
Summary
As !2 (merged) has gotten merged, some more work on top of that would be needed. It would be great if the document containing all of the versions would be generated from an ".rst" file using sphinx, rather than using raw HTML for that as mentioned in Tristan's comment here.
Something also worth mentioning is removing the "About" section on the main page, although I would say this is not yet set in stone, so it might be worth asking on IRC / discussing on this issue whether we want to do that or not.
Acceptance criteria
The CI uses reStructuredText in order to generate the HTML linking to all of the other versions' documentation, preferably keeping the current behaviour (e.g. fetching everything and building the file automatically, with as least manual work as possible).