Improve SEO for Flax

Github Discussions doesn't always seem to work well with Flax, which means some questions that are answerable do not show up in Google search.

Here is an example: Google search for "How to count the parameters in a Flax model?" returns just the Flax Basics doc, and some unrelated questions, while we have a Github Disucssions with _exactly_ this title: [How to count the parameters in a Flax model?](https://github.com/google/flax/discussions/1224)