Consider using sphinx-autosummary-accessors for accessor autosummaries

[jthielen]

While I don't think that our current accessor documentation is that bad at this point, it may be worth considering using the upcoming sphinx-autosummary-accessors extension to standardize around how other libraries like xarray and pandas are doing it (with a more collapsed table that links to full documentation for each method, which makes it easier to browse).

This extension would require Sphinx >=3.1 for optimal use however, so nothing would be able to be done with this until #1415 is resolved.

[dopplershift]

Sounds good. I'm working #1415 right now, it looks like it should be an easy transition actually--surprising the hell out of me.