chore(python): autogenerate docs/index.rst

[parthea]

This PR is a first step towards auto generating docs/index.rst for most python repos.

The function detect_versions in synthtool/languages/python.py is identical to the one used by node here so alternatively, we could consider moving the existing function detect_versions outside of node.py.

[parthea]

As an example, I ran the version of synthtool in this PR in the python-memcache repo and auto generated the docs/index.rst file. You can see the result here.

[dandhlee]

Seems like from Sphinx side not much has changed, I'll take that as a good thing. LGTM.

[bcoe]

Not a blocker for me on landing this, but ideally we'd DRY up the detect version logic.

If you don't opt to do so in this PR, mind creating a tracking issue to do so?

[bcoe]

How hard would it be for us to pull this into a common file, if the logic is identical?

[parthea]

Done. Please could you take another look?

[bcoe]

Thank you for cleaning this up @parthea 👏 , reading through the current owlbot logic, I saw that versions is actually loaded in a separate code path:

        # Collect the subdirectories of the src directory.
        src = Path("src")
        versions = [v.name for v in src.iterdir() if v.is_dir()]
        # Reorder the versions so the default version always comes last.
        versions = [v for v in versions if v != default_version] + [default_version]
        logger.info(f"Collected versions ${versions} from ${src}")

So I think you've gotten rid of some code that would have otherwise have hung around indefinitely.