docs: Add new OLX example course

[sarina]

Minimal OLX course, showcasing various problem types, HTML units, video, custom tabs, grading policy. Imports into Studio. Omits certain Studio artifacts that aren't critical to basic course functionality (but could be added in later, if there's appetite for making additional documentation).

[ormsbee]

Really off-the-cuff comment: The OLX url_names are case-sensitive, so I think that we should encourage people who are slinging OLX by hand to use lower case and underscores for their filenames/usage-keys. Otherwise it's really easy to slightly miss the capitalization on mixed case references. For instance, "LTIintroduction.xml‎" lowercases the "i" in "introduction", but "OLXResources.html‎" capitalizes the "R" in "Resources". I don't think we have great error messaging when you have things like this, though there may have been work there on the import linting side of things that I'm not aware of (I believe Arbisoft folks folded in an MIT library to validate OLX on import, but I don't remember any of the details).

[sarina]

Add a multipart problem

[kdmccormick]

LGTM pending the things we discussed in our meeting, which you've captured in comments 🚀