[TRACKING] Improve Contributing Guidelines

Currently, the NRC repository lacks custom guidelines for contributors, which leads to inconsistency on issues/PRs and manual toil on maintainers.

We may consider:

• Adding Issues templates/forms
• Implementing a standardized label schema (kind/ area/ priority/)
• Customizing CONTRIBUTING.md (including local setup & dev environment)
• Document available tests, add instructions for writing tests setting up ProwJobs

[edit: a few changes were deemed not necessary after discussion]
- [ ] Improving PR templates
- [ ] Standardizing commit messages (e.g., Conventional Commits)
- [ ] Adding policies for Branch naming
- [ ] Defining Merge/Squash policy for clean history

Adding guidelines/policies on how to contribute helps contributors have a clear path and facilitates automating chores like release notes, changelogs and triaging. It also makes it possible to provide contributors instant feedback through CI checks.

Review the existing CONTRIBUTING.md, issue templates/forms, test documentation, and ProwJob setup instructions to identify what is already covered. Define the contribution workflow and label or testing guidance that the repository lacks, then verify that the resulting documentation gives contributors a clear local setup, testing, and issue/PR path.