[llvm] [Github] Add Pull Request template with links to Github docs/Discord/Forums (PR #71923)

[Tom Stellard via llvm-commits]

tstellar wrote:

> > Will this hidden comment end up in the commit message?
> 
> I'll check this on a fork. It's possible that a merge from the command line would include it by default.

You should also test a 'squash and merge' from the UI.

https://github.com/llvm/llvm-project/pull/71923