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.
@explorerleslie this is a bookmark for cross-references to point to. It doesn't appear in the rendered HTML or PDF. Including punctuation in the bookmarks makes them lose identifying formatting in rst, which is why I take it out for this purpose only.