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/

I'm not sure exactly what this means.

If my account completes 10 problems during a particular week it will increase both the "active" and "problem" counts by 1.

Internally we call them activity categories, since a particular action can belong to multiple categories.