DOC: document subclasses in API docs with selection of specific methods/attributes
xref https://github.com/pandas-dev/pandas/issues/18147

OK, https://github.com/pandas-dev/pandas/commit/500ab71bda109327cf191a11fcf514eaffd70167 removes our hack in the vendored `numpydoc/numpydoc.py` to exclude autosummary generation for certain methods. Instead, we'll explicitly list the relevant methods in a new `Methods` section in those docs. This let's us include things like `IntervalIndex` in the API docs, without having all the regular index methods in the table, and without auto-gen making pages for all of those.

The downside is that we have to list the methods in `Methods` in the docstring and under a toctree in `api.rst`. But that's not so bad.

https://github.com/pandas-dev/pandas/commit/257fc8a0af7de854d0f5e2f0bbc9380320760c69 is removing all the warnings about references a non-existent document / this document isn't included in a toctree. But it's a bit tedious so I'll come back later (~600 warnings remaining :/ )

