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

[David Spickett via llvm-commits]

DavidSpickett wrote:

See https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository for how the template works. From my experience over in Compiler Explorer, it gets added to the end of the text Github initially selects (usually the commit body).

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