WIP:  Content Experiments Documentation

See draft documentation at:  http://draft-nested-components.readthedocs.org/en/latest/organizing_course.html#nested-components
are there any shades of gray? for example, is it ok-ish to change a group configuration in a unit that has not been released yet, or that is hidden to students? are there more issues to consider if you change the group configuration in a released & public unit?

xref to procedure for actually doing this.