Auto-generate DataFrame, and other member docs

[sethmlarson]

Currently we have a lot of files under docs/source/reference/api and all of them are basically the same. It'd be nice to have an auto-generation step to create these files the same way that Pandas does.

Steps to generate:

• Look at each class
• For each public member create a file rst
• Within the rst file choose autofunction / autoproperty etc defining on what type of thing the member is. inspect module will be useful here.
• Make this step a part of the docs task.

[sethmlarson]

There might be something to do here with autosummary and sphinx.ext.autosummary