Render API docstrings in .py files as html

[rgommers]

This does the following:

• Ensure the package in spec/API_specification/dataframe_api/ is importable
• Ensure all docstrings for objects/methods are rendered in the html docs via autodoc
• Add a description for operators supported by the dataframe object (and a few TODOs)
• Adds Sphinx templates for styling the html rendered versions of methods and attributes properly (taken over from the array-api repo)
• Adds some infrastructure for static typing that we may need. So far only the Scalar class needed a TypeVar, but there should be more to do there.

[jbrockmendel]

should column be capitalized?

[rgommers]

I didn't do that on purpose. Maybe one thing to discuss in the context of constructors is whether the name matters (Column, DataFrame, etc.). For array libraries we left this out on purpose, because it's never needed and existing libraries name these things differently. The same could apply here, unless you allow calling object.__init__ as a constructor.

[jbrockmendel]

a few nitpicks, otherwise looks fine

[rgommers]

CI is happy and comments are addressed, so I'll go ahead and merge this.

[rgommers]

CI for deploying is happy, rendered docs are up on https://data-apis.org/dataframe-api/draft/API_specification/index.html.