Migrate OLx Documentation to docs.openedx.org

[e0d]

Currently the OLx docs seem to only exist here:

https://edx.readthedocs.io/projects/edx-open-learning-xml/en/latest/what-is-olx.html

I think they are still broadly useful and should be migrated to new site.

[feanil]

@jswope00 is this on your list already, if not, would you mind taking a stab at moving these docs?

[jswope00]

@e0d @feanil It wasn't in the initial migration because it didn't fall neatly into the "educators" category, but I agree it should be moved and I've got the experience now with these moves, it'll be quick. I'll take care of it within the next week.

[jswope00]

Note to self: Fix the external link in source/educators/references/course_development/create_problem.rst that goes to legacy docs. Repoint it to Open edX docs once this migration is done.

[jswope00]

Completed: #819