Improve navigation in Reg Admin page (especially for large competitions)

[timreyn]

Is your feature request related to a problem? Please describe.
https://www.worldcubeassociation.org/competitions/WC2025/edit/registrations is quite long -- 141 pages printed! Finding the sections (e.g. wait list) takes quite a while, and involves lots of scrolling. This applies to other competitions as well, of course it's less prominent there though.

Describe the solution you'd like
One option would be to have links along the left side bar, under "Registration", one of each of the registration sections, that navigate to the right part of the page. This would only be visible when the registration page is open.

Ugly mock:

[dunkOnIT]

Other ideas here are:

• collapsible lists
• only load n items per list and paginate [with a filter/search bar to make it easy to find a specific registration]