DOC: api display
our API page shows lots of useful information: http://pandas-docs.github.io/pandas-docs-travis/api.html and is pretty browsable.

But the toctree is mostly useless and encumbered by ``pandas.Sereies.rdiv`` which is pretty long (these tend to wrap).

So ideas to fix are:

- limit toc tree depth (``:maxdepth: 2``) (we set to 4 higher)
- try to find a way to control the title on the toc tree items
