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.
believe it or not, the edit checking that takes place after you click Save Changes does it for you. Since at this point they may or may not have other policy keys in this dict, I chose to not talk about the comma at all rather than go into the "if there are more keys, add a comma, but if there aren't you don't need one" routine. I can add if you think it's less confusing.