docs: add release notes contributor guide

[DanielBlei]

What this PR does / why we need it:

Document the release-note block process: when a note is required, how to apply it in a PR, and how to write a good one.

This doc will become the target of the Prow release-note plugin's configurable guidelines URL (kubevirt/project-infra#4189).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
N/A

Special notes for your reviewer:

A few other homes were suggested in the issue (user-guide, kubevirt/kubevirt docs, sig-release). I went with this repo as a starting point since the plugin URL is a single global setting, linked from bot comments across many repos in the org.

This documentation follows the same pattern as the k8s release notes guide, which lives at kubernetes/community/contributors/guide/release-notes.md. That said, I'm happy to move it to another place.

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR.
Approvers are expected to review this list.

• Design: A design document was considered and is present (link) or not required
• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Testing: New code requires new unit tests. New features and bug fixes require at least on e2e test
• Documentation: A user-guide update was considered and is present (link) or not required. You want a user-guide update if it's a user facing feature / API change.
• Community: Announcement to kubevirt-dev was considered

Release note:

[kubevirt-bot]

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[kubevirt-bot]

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