DOC: add full API reference using Sphinx recursive option

[rbnvrw]

I was trying to build the docs locally and found that we do not have an overview of the full trackpy API. This adds a toctree at the bottom of the API page where you can click through all files / modules and functions. I think it is a useful addition because also the less often used functions are browse-able through the docs in this way.

[caspervdw]

Thanks @rbnvrw . I’ll merge, but let’s have a close look at the result and see what functions we actually want to document. Not all is part of the external API.