Restructure the API section of the docs

[drbenvincent]

Right now the structure of the API docs is very flat, in that all functions and classes within a given module appear on the same page. This does give rise to quite a lot of scrolling and an increased sense of clutter.

Perhaps better ways to structure the API docs are seen in pymc-marketing or pymc where there is more hierarchy and each function or class has it's own page/url.