Add ability to easily change documentation version

[emergenz]

Currently, it is not easily possible to change the flax documentation to older versions. However, this is particularly useful when working with legacy code bases, especially in the light that the Flax API has changed quite substantially.

Problem you have encountered:

I can only change between latest and v0.5.3, but I need to read older docs to understand legacy codebases.

What you expected to happen:

I expect to be able to change to older versions of the documentation, like how it is possible in the huggingface transformers documentation.

Steps to reproduce:

Go to https://flax.readthedocs.io/en/latest/ and click on v: latest.