[docs] Document regular LLVM sync-ups

This documents current regular LLVM sync-ups that are happening in the
Getting Involved section.

I hope this gives a bit more visibility to regular sync-ups that are
happening in the LLVM community, documenting another way communication
in the community happens.
Of course the downside is that this is another location that sync-up
metadata needs to be maintained. That being said, the structure as
proposed means that no changes are needed once a new sync-up is added,
apart from maybe removing the entry once it becomes clear that that
particular sync-up series is completely cancelled.

Documenting a few pointers on how current sync-ups happen may also
encourage others to organize useful sync-ups on specific topics.

I've started with adding the sync-ups I'm aware of. There's a good
chance I've missed some.

If most sync-ups end up having a public google calendar, we could also
create and maintain a public google calendar that shows all events
happening in the LLVM community, including dev meetings, sync-ups,
socials, etc - assuming that would be valuable.

Differential Revision: https://reviews.llvm.org/D98797