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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> @tstellar do you know where llvm-bot's code is? Is it possible to run an instance of it on your own fork?

I found it in `llvm/utils/git/github-automation.py`.

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