[docs] Document and publish LLVM community calendar

Let's introduce and publish an LLVM community calendar.

The idea is that organizers of events such as online sync-ups or office hours
invite calendar@llvm.org to the event they're creating. That way, the calendar
publicly visible at
https://calendar.google.com/calendar/u/0/embed?src=calendar@llvm.org will show
the event.

The hope is that having a single calendar showing all LLVM events makes it
easier for both new comers and experienced people to discover events they're
interested in.

This patch partially implements https://github.com/llvm/llvm-project/issues/55426

We could also give pointers to the calendar in a few other places, e.g. from
the main LLVM page, but let's introduce the incrementally.

Differential Revision: https://reviews.llvm.org/D127852