The form of pySpinw API doc

[AlbertRyu]

I've switched the API doc generation from :autodoc to :autosummary, which now provides a summary table for each submodule.

I also experimented with grouping the docs by category (SpinW/pySpinW#201) — the result looks cleaner than the original alphabetical ordering.

Should we support both views (alphabetical + categorical), or will categorical grouping suffice?

[mducle]

I think just having the categorical groups is fine. We don't have so much classes/methods that we need an alphabetical list - there is also a search functionality if people want a specific class or method.