Reorg of cohorts and discussion topics

Pulled the reorg of cohort and discussion topics out of previous branch DOC-1077 so that it is no longer dependent on that open source PR. Topics about setting up and managing discussion now separated from the cohort configuration section.
@mhoeber @lamagnifica @srpearce @explorerleslie please give thumbs if you're OK with the reorg.
Would like to create new doc for cohorted courseware on top of this file organization.
Taken out of another file, but restructured (was parallel with the procedure for making inline discussions uncohorted -- but that is blocked and now in another PR).