Set up Artifact Registry project repository

Context

The Artifact Registry project repository has been created at https://gitlab.com/gitlab-org/ops/artifact-registry. This issue tracks the remaining setup steps per the GitLab Repositories checklist.

Project scaffolded using https://gitlab.com/gitlab-com/gl-infra/common-template-copier.

Checklist

• Add to projects.yml (MR: gitlab-com/www-gitlab-com!142736 (merged))
• Add LICENSE (EE, confirmed by #legal)
• Add CONTRIBUTING.md with DCO + License section
• Add README.md (via common-template-copier)
• Add CODEOWNERS file (via common-template-copier, group: https://gitlab.com/gitlab-org/maintainers/ops/artifact-registry)
• Enable merge trains
• Enable merge only if pipeline succeeds
• Enable merge only when all threads resolved
• Disable users can request access
• Set up CI/CD config (via common-template-copier)
• Add to projects_part_of_product.csv (MR: gitlab-data/analytics!13472)
• AppSec categorization (MR: gitlab-com/gl-security/product-security/inventory!435)
• Enable security scanners (via standard-build CI component)
• Onboard to Renovate (via common-template-copier)
• Set up security mirror (MR: gitlab-com/gl-infra/infra-mgmt!2518)
• Baseline MR approval rules and protected branch settings
• Create maintainers group (gitlab-com/team-member-epics/access-requests#42240)