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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> I'm wondering if it would be worthwhile to directly add some info about how to add reviewers here, rather than refer to the docs, given that most newcomers seem to get confused about not being able to set reviewers in the github interface?

Sure, the very frequent queries could be included.

> Another thought (not sure if useful) is that I've seen in other projects that some add a check-list here, to help people remind what checks they should've done before or shortly after uploading their PR.

My only issue with that is whether we have to remove that text before landing, and who does that. Though it could go in this same comment block, it just wouldn't be interactive. Which is fine if it's a reminder, not something reviewers are gonna require is done.

I'm just wary of adding a manual step that people may forget to do, but also I've not been a merger on a project with one of these checklists either so don't have the answers here.

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