Duo Review: Detect duplicate/missing frontmatter keys

Why is this change being made?

💡 Provide a detailed answer to the question on why this change is being proposed, in accordance with our value of Transparency. Please add the details saying why, not just what in this section.

Slack discussion with duplicate frontmatter keys, causing errors with local builds.

This MR adds a Duo Review custom instruction to let MR authors know about duplicated/missing frontmatter keys.

AI-generated summary

This change adds a new validation rule to the code review process that automatically checks markdown files for proper formatting of their metadata sections. The new rule ensures that files have the required information (like title and description), prevents duplicate entries, validates that dates are in the correct format, and catches common spelling mistakes in metadata field names. This helps maintain consistency and quality across documentation files by catching these issues during the review process before they get merged into the main codebase.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to a code owner, typically a handbook maintainer
  • If you're unsure, please mention the handbook owner
• For transparency, share this MR with the audience that will be impacted
  • #handbook Slack channel if there is any user impact
  • #mr-buddies Slack channel if changes may impact more users
  • #whats-happening-at-gitlab linking to this MR if a large number of team members are impacted

Commits

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