Improve autogenerated SDK documentation

[negz]

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.

[negz]

FWIW I went with pdoc rather than Sphinx or mkdocs to start with because:

• They both wanted to embed generated reference docs inside some broader docs, which I don't know that we need.
• They both seem like 500lb gorillas. I don't want to learn and deploy a whole software project just to have documentation.

[negz]

https://github.com/googleapis/protoc-docs-plugin - I see this exists, but is archived.