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

[Kristof Beyls via llvm-commits]

kbeyls wrote:

I was thinking that the check list at least might be useful for more than just new users.
I think it would also be useful for occasional users. Or maybe even for regular contributors.
I would be surprised if it would not result in better quality PRs, resulting in slightly less pressure on reviewers to point out easy to fix issues.

So, I think I wouldn't try too hard to only add this for new users, assuming it also has useful information for occasional, or even frequent, PR creators.

Having a bot create such a comment may well provide at least half the value of what's intended to be achieved by https://github.com/llvm/Community.o/issues/7.

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