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

[Kristof Beyls via llvm-commits]

kbeyls 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?

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:

- [ ] I considered adding or adapting tests for changed or new functionality.
- [ ] the regressions tests all pass locally on my machine.
- [ ] my patch is properly formatted using e.g. clang-format.
- [ ] I found and added appropriate reviewers on the PR, either by mentioning them in a comment or in the "reviewer" widget.



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