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.
"Scroll back to the **Cohort Configuration** field to verify that your entry was saved as you expect. Entries that do not contain all of the required punctuation characters revert to the previous value on save, and no warning is presented."