Add versioning to the docs

[allevitan]

As we start preparing releases, we need to think at some point about keeping versions of the docs around for older code versions. It's not critical now, but we need to have it in place before publishing anything on the code, I think. See #38 for a bit more discussion

[gnzng]

I started doing a little bit of research and test runs, it seems that sphinx-multiversion provides that: https://www.codingwiththomas.com/blog/my-sphinx-best-practice-for-a-multiversion-documentation-in-different-languages