Improve autogenerated SDK documentation

### What problem are you facing?

We have autogenerated SDK documentation at https://crossplane.github.io/function-sdk-python/function.html, but it's not great.

Some things that could be improved:

* There's no versioning - the generated docs always and only reflect the `main` branch.
* The landing page is empty.
* There's no documentation for the protobuf generated code.

Documentation for the protobuf generated code seems like it would be very valuable, since there's not much else to this SDK apart from a few utilities. Unfortunately the code itself doesn't seem to have docstrings, despite the protobuf messages it's generated from having them.

### How could this Function help solve your problem?

* Figure out if there's some way to copy docstrings from the protobuf schema to the generated code.
* Use a better docs tool (Sphinx, mkdocs, etc). Ideally something low maintenance.