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

[Tobias Hieta via llvm-commits]

tru wrote:

> 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. For LLVM, off the top of my head, I'm thinking of e.g. something somewhat as follows:

My experience with the checklists are that most people ignore them. But I don't mind them being there, we just need to make sure that it's updated if we change the documentation to suggest something other.

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