Add pull request CI triggers

[jacobtomlinson]

Currently PRs from forks do not trigger CI. This PR adds those triggers.

• Update CONTRIBUTING.md with instructions for maintainers on running CI for forks
• Added a workflow that comments on PRs to explain to users why CI isn't running, and to point maintainers to the docs
• Add comments to workflow files to explain why we aren't triggering on PRs

[jacobtomlinson]

Ah I see this is a limitation of NVIDIA hosted GitHub Actions runners. The solution is for a maintainer here to pull a PR locally once they confirm is is benign and push it back up to a branch that matches the pattern pull-request/[0-9]+. When the CI for that branch completes it will mark the PR with the results due to the commit SHAs matching.

I'll revert the changes in this PR and add some docs on this process instead.