docs: Update mirrors documentation with Release Platform module

Summary

Updates the security mirrors documentation to recommend the new YAML-driven Release Platform module as the preferred method for creating security mirrors.

Changes

• Added Release Platform module section as the preferred method for creating security mirrors
• Wrapped legacy Terraform instructions in a collapsible <details> block
• Added "Importing Existing Projects" section explaining current limitations
• Updated prerequisites to reference the new import section
• Added PSIRT approval to security mirror requirements

Why

The Release Platform module (work item #50) provides a standardized, YAML-driven way to provision the complete 3-mirror architecture with built-in integrations for common-ci-tasks, Vault, and more. This simplifies onboarding new projects and ensures consistent configuration.

References

• Release Platform module: https://gitlab.com/gitlab-com/gl-infra/terraform-modules/gitlab/release-platform
• Configuration file: https://gitlab.com/gitlab-com/gl-infra/infra-mgmt/-/blob/main/data/projects/release-platform/repos.yaml
• Related epic: gitlab-com/gl-infra/software-delivery#50 (closed)