Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
This list feels a little oddly placed, because there are a bunch of extra notes before it. When I first started reading this list, I was expecting to see instructions for how to download profile information, since that's what came right before the list. I would suggest moving this towards the beginning of the subsection or adding some kind of introductory text.