Do something better with conference and workshops subdomains

The following URLs are served from dedicated repositories:

* https://conference.imagej.net/
* https://workshops.imagej.net/

They contain _materials_, but not an overview, for conferences and workshops respectively.

Unfortunately, because the repositories are located at imagej/conference and imagej/workshops respectively, the following URLs just redirect to the above subdomains:

* https://imagej.github.io/conference
* https://imagej.github.io/workshops

This is confusing, because:

* Conferences are overviewed at https://imagej.github.io/events/conferences
* Workshops are overviewed at https://imagej.github.io/learn/presentations#workshops

We should consider shuffling this content somehow so it's less confusing. Here are is one solution that occurs to me (using imagej.net in all cases, since that's what will serve imagej.github.io content within the next few weeks):

1. Rename imagej/conference repository to imagej/conference.imagej.net, and imagej/workshops repository to imagej/workshops.imagej.net, so the CNAMEs still work, but imagej.github.io/(conference|workshops) won't serve these other repositories anymore.
2. Move https://imagej.net/events/conferences/* to https://imagej.net/conferences/, and move https://imagej.net/events/presentations#workshops to https://imagej.net/workshops/*
* And move events/presentations to /presentations
* And put a link at the bottom of the presentations page to /workshops
3. Make https://conference.imagej.net/ and https://workshops.imagej.net/ top-level links respectively redirect to https://imagej.net/workshops and https://imagej.net/conferences, which now serve those overview pages.
4. Move the old conference.imagej.net/index.html and workshops.imagej.net/index.html to imagej.net/conferences/materials.md and imagej.net/workshops/materials.md pages respectively in this repository, styling them consistently with the rest of the site.

I think that covers all our bases, while making everything much simpler to understand?

Start by comparing the imagej/conference and imagej/workshops repositories with the events/conferences and learn/presentations pages named in the issue. Trace the existing index.html files, CNAME behavior, and linked URL paths. Done means the content locations, redirects, repository names, and overview/materials pages have an agreed, consistent structure.

Written by the indexing model from the issue text.