[llvm][Docs] add discord bot calendar capability notes

[gburgessiv]

The LLVM Discord bot now has the ability to scrape the LLVM calendar & send reminders about upcoming office hours events and sync-ups. Document that here.

While I'm in the area, add a note about the bot's ability to @mention people when they're on buildbot blamelists.

Related to llvm/Community.o#19

[AaronBallman]

Thank you for working on this! I think it looks good, but please wait a bit before landing in case anyone else has feedback.

[kbeyls]

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.

[gburgessiv]

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!

[kbeyls]

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!

[gburgessiv]

Thanks for the reviews!

Ignoring buildkite failure because this change shouldn't impact LLVM.tools/llvm-gsymutil/ARM_AArch64/macho-merged-funcs-dwarf.yaml :)