Document copy-PR testing authorization (#2423)

### What does this PR do?

Type of change: documentation.

Explain how authorized vetters start NVIDIA-runner checks for pull
requests from forks, including the SHA-qualified copy-PR command and
links to the centralized contributor and vetter guidance.

### Usage

N/A; documentation-only change.

### Testing

- `pre-commit run --files CONTRIBUTING.md`
- `git diff --check`
- Verified both copy-PR documentation links resolve

### Before your PR is "*Ready for review*"

- Is this change backward compatible?: N/A
- If you copied code from any other sources or added a new PIP
dependency, did you follow guidance in `CONTRIBUTING.md`: N/A
- Did you write any new necessary tests?: N/A
- Did you update `CHANGELOG.rst`?: N/A
- Did you get Claude approval on this PR?: N/A

### Additional Information

Clarifies the missing required-check state encountered on #2231.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Documentation**
- Updated fork pull-request guidance to require an authorized reviewer’s
approval before NVIDIA-hosted CI runs.
- Added instructions for contributors without write access to obtain
GitHub workflow approval from a reviewer with write permission.
- Added the `/ok to test <full-head-sha>` command for authorizing
testing against a specific commit.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: realAsma <akuriparambi@nvidia.com>

CONTRIBUTING.md

@@ -264,6 +264,9 @@ nox -s "unit-3.12(torch_211, tf_latest)"
 ## Submitting your code
 
 - Submit a pull request and let auto-assigned reviewers (based on [CODEOWNERS](./.github/CODEOWNERS)) review your PR.
+- **Fork PRs:** An authorized vetter must comment `/ok to test <full-head-sha>` to run NVIDIA-hosted CI.
+- **Contributors without write access:** When GitHub requests workflow approval, a reviewer with write permission
+  must select **Approve and run workflows** to start GitHub Actions CI.
 - If any CI/CD checks fail, fix the issues and push again.
 - Once your PR is approved and all checks pass, one of the reviewers will merge the PR.
 - If your PR is a bug fix that should also land in an ongoing release branch, add the