Clean up the html docs for the dataframe API standard

[rgommers]

This culls some boilerplate and brings a bit of method to the autodoc madness. All GroupBy, Column and DataFrame methods are now automatically included, and the front page of the API specification is much easier to read.

Finally, removes two empty pages (benchmarking and usage data) that we are unlikely to fill any time soon.

[rgommers]

The rendering wasn't great here compared to what I had locally, but unfortunately between Sphinx, docutils and myst-parser it's almost impossible to find a warning-free or bug-free version. Myst has a pretty bad bug where cross-references between .md docs no longer work: executablebooks/MyST-Parser#749. Causing this:

/home/circleci/repo/spec/use_cases.md:7: WARNING: local id not found in doc 'purpose_and_scope': 'Goals' [myst.xref_missing]
/home/circleci/repo/spec/use_cases.md:7: WARNING: local id not found in doc 'purpose_and_scope': 'Scope' [myst.xref_missing]
/home/circleci/repo/spec/use_cases.md:10: WARNING: local id not found in doc 'purpose_and_scope': 'Stakeholders' [myst.xref_missing]

For now I'll drop these cross-links to get to green.

[rgommers]

Better - preview in Circle CI looks quite nice now.