Ahodges/doc/doc 792

Initial, proof of concept draft, focusing initially on enrollment chapters. @shnayder , @clintonb , @dsjen , @mulby , @brianhw , @rocha , @JAAkana , @WatsonEmily , @stroilova please feel free to comment anytime on this PR from now through release.

An HTML draft is now available! http://draft-edx-insights.readthedocs.org/en/latest/

At the moment, I believe that engagement metrics are determine weekly and the others are determined daily. I have lost track of the timing/expected schedule for any of the updates however.
@mulby I'm going to add a doc task to check this with @shnayder , which may then result in both doc changes and a new analytics task. DOC-973