[llvm] [llvm][Docs] add discord bot calendar capability notes (PR #122140)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> > Add some text to the section https://llvm.org/docs/GettingInvolved.html#guidance-on-what-to-put-into-llvm-community-calendar-invites, as probably that's the place where people look for guidance when setting up a new event
> 
> Huh, I didn't see that section. Agreed that it'd be good to add stuff there; PTAL!

Thanks, fully LGTM now!

https://github.com/llvm/llvm-project/pull/122140