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.
yes, bulleting out the info in the 2nd paragraph (in response to Christina's comment earlier), and moving up the note (which was previously after these steps has overloaded the intro and diluted the context. I'll see if I have time to rework this again.