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

[David Spickett via llvm-commits]

DavidSpickett wrote:

If this is the case, maybe we can use the same text as an llvmbot comment for PRs from new users (istr Github itself knows who is a new contributor).

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