Cohorted courseware: Reword cohorts and content experiment overview/config in preparation

Preparation for cohorted courseware doc: separate out the conceptual and configuration content that applies to cohorted content groups vs content experiments; update terminology for the redesigned group configurations page ("experiment groups").


Yes, I considering removing several unnecessary images in this section, but leaving it until I finish the new cohorted courseware changes, which will have some impact on the group configuration page.