Event horizon feature

This is amazing (https://github.com/galaxyproject/training-material/pull/4963)

I'd like to have this on the main single-cell topic page. I tried (and failed) to do this. ALTHOUGH, this is determined by using the materials, whereas stuff like the CoFest I'm running has lots of options so I suppose is only there as an event because I've tagged it with single-cell, rather than because it's using the tutorial. Hmm...

If you pull this over for me, I'll make an FAQ based on the PR you write to do it if you @ me!

Review pull request #4963 and the main single-cell topic page first, then determine how the event horizon content is selected from training materials. Done means the feature from that PR appears on the single-cell topic page and the change is clear enough to document in an FAQ.

Written by the indexing model from the issue text.