WIP: Documentation for Publishing Changes

I'd combine this with Unit Publishing Status and add information about what the instructor sees. In other words:

6.5.6. Unit Publishing Status and Visibility

Units can have one of the following statuses:

- Draft (Never Published): You see the version of the content that you're working on. Students never see a unit with this status, even if the release date has passed.
- Published and Live: The release date for the section and subsection have passed. You've published the unit and haven't made any changes to it. You and students both see the current version of the unit.
- Published (Not Yet Released) *note the change from "and not yet released")*: You've published the unit, but the release date hasn't passed. Students cannot see this unit until the release date passes.
- Draft (Unpublished Changes): You see the draft of the content that you're working on. If the release date has passed, students see the last published version of the unit. If the release date hasn't passed, students can't see your content.
- Visible to Staff Only: Students never see a unit with this status, even if it has been published and the release date has passed.