docs: Add comprehensive contributor ladder documentation

[Copilot]

This PR introduces a comprehensive contributor ladder documentation to provide clear guidance on how community members can grow from users to maintainers within the Gatekeeper project.

What this PR adds

CONTRIBUTOR_LADDER.md - A detailed 200+ line document that defines:

• Clear role progression: Community Member → Contributor → Maintainer
• Specific requirements for each role with technical and community expectations
• Advancement processes with consensus-based decision making
• Gatekeeper-specific criteria emphasizing security awareness and policy impact understanding
• Professional emeritus process for maintainers stepping down
• Inactivity management to ensure project health

Key features

The ladder is tailored specifically for Gatekeeper as a Kubernetes policy controller, emphasizing:

• Deep understanding of Kubernetes admission controllers and security implications
• Experience with OPA/Rego and policy enforcement
• Sound judgment regarding changes that impact cluster workloads
• Community leadership and mentorship expectations for maintainers

Documentation integration

Also updates:

• CONTRIBUTING.md to reference the new ladder
• website/docs/help.md and website/versioned_docs/version-v3.20.x/help.md to include ladder link

The ladder builds upon the draft work from PR #3559 while incorporating best practices from the Kubernetes community contributor ladder structure, adapted for Gatekeeper's specific needs as a security-focused project.

Fixes #3434.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.