[llvm] [docs] Document which online sync-ups are no longer happening (PR #89361)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> Thanks for updating the list. Should we have a criteria that if any meetings haven't had N occurrences then they can be safely deleted?

I'm not sure what a good rule of thumb would be.
Maybe for now, let's land this PR and get the framework set up to indicate which online sync-ups are no longer happening. We can refine guidelines on when to move online sync-ups from "active" to "past" as we gain more experience.

https://github.com/llvm/llvm-project/pull/89361