docs: add Docker Image Security Guide for cosign verification and deployment best practices (#25439)
- New doc page covering all signed image variants, verification commands,
CI/CD enforcement (K8s Sigstore Policy Controller, GCP Binary Authorization,
AWS/EKS, GitHub Actions), digest pinning, and safe upgrade patterns
- Added to sidebar under Setup & Deployment
- Cross-linked from the existing deploy.md cosign section
Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Krrish Dholakia <krrish-berri-2@users.noreply.github.com>
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