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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Added how to get reviewers and how to ping based on the text in https://llvm.org/docs/Contributing.html#how-to-submit-a-patch.

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