docs(SECURITY.md): expand security policy for CRA compliance

[gundalow]

Summary

• Replaces the single-line SECURITY.md with a complete security reporting template aligned with the Ansible Security Policy and Vulnerability Management Policy going live at ansible.com/security
  • NOTE this URL currently points elsewhere, it will be updates once add: Ansible Security Policy ansible/ansible-documentation#3721 is merged
• Adds structured vulnerability reporting guidelines (9 fields matching the security policy), response timeline, backport policy, and a link to the full policy
• Adds the EU Cyber Resilience Act (CRA) open-source steward statement as required for Red Hat-stewarded projects

Related PRs

• add: Ansible Security Policy ansible/ansible-documentation#3721 — Ansible Security Policy
• add: Vulnerability management policy ansible/ansible-documentation#3734 — Ansible Vulnerability Management and Incident Response Policy

Test plan

• Verify all 9 submission fields match security_policy.rst once add: Ansible Security Policy ansible/ansible-documentation#3721 merges
• Verify ansible.com/security resolves once the documentation deploys
• Confirm CRA steward statement text with legal/compliance

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