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.
Maybe "only includes" instead of "supports"? I'm tripped up because "supports" sounds like "gives you the option to", which isn't the case here -- it forces you to have conversations that are specific to the members of the cohort.