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").


The title of this section ended up being "Setting up Discussions in Courses with Student Cohorts" to avoid "cohorted". Suggest changing the literal string for the xref to that phrasing.