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

[David Spickett via llvm-commits]

DavidSpickett 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.

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