Versioned documentation

[uniqueg]

It would be nice to have a reference documentation for each API release rather than only having a single documentation version that points to the latest release (or, as it is currently the case, not matching any release at all; see #191).

I am not familiar with the build system for the docs, and whether hosting multiple documentation versions is possible with that system, but for what it's worth, here is how Read the Docs supports multiple documentation versions: https://docs.readthedocs.io/en/stable/versions.html.

If versioning documentation is not feasible, it should be clarified (and ensured) that the reference documentation always points to the latest release, and the actual commit/tag should be explicitly linked to in the documentation.