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

[Kristof Beyls via llvm-commits]

https://github.com/kbeyls approved this pull request.

Looks good overall.

I just have 2 suggestions for a bit more text:
1. in the comment I did inline.
2. 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. I'd consider adding an example meta-data text block at the end of the example there, as I assume many new organizers might simply copy paste that example and then adapt it. By doing so, we'd make it a lot more likely for this bot functionality to be used by most events.

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