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.
Remove all references to the "auto_cohort" key. It is no longer used. Automatic cohorting will always happen (with a default cohort) if a student visits the discussion forums and he/she has not been assigned to any cohort.