Make structure of edu-nexus more transparent

Currently, educational nexus is full of great resources ... but very long. There are broader restructuring conversations, but we could temporarily add sub-headings that ease navigation.

For example:
- Teaching materials
- - Syllabi
- Reference sources
- - Glossary
- Educator training
- - Educators Corner

...

However, this requires someone who understands each of the menu items to come up with a useful clustering- @flavioazevedo any suggestions for who could do this?

Technically, sub-headings can be menu items without URLs; we can make them stand out by adjusting the menu.html partial - [see ChatGPT guidance](https://chatgpt.com/share/67a3c9b1-9b68-800e-bb84-80673edcd4fd).