Improve release notes index generation

[danroth27]

Summary

• make creation of the reader-facing milestone index an explicit release-notes skill responsibility
• replace the loose README example with the established release index structure used by the early .NET 11 previews
• require every generated component file to be linked exactly once in consistent product groupings
• keep build versions and VMR provenance in build-metadata.json instead of the public index
• use stable documentation and download links, with the exact milestone landing page only after artifacts publishing creates it

Validation

• Markdown lint and link checks pass for the changed skill files
• repository Checkov scan reports the existing CKV2_GHA_1 failure in .github/workflows/copilot-setup-steps.yml
• repository-wide Prettier reports existing formatting warnings; the changed Markdown also follows the repository's existing non-Prettier style