Add documentation for MR title template feature (Part 4/4)

Marc Shawrequested to merge
mr-title-template/4-docs into mr-title-template/3-api-ui

Summary

Part 4 of 4 for customizable MR title templates.

Adds user-facing and API documentation for the feature.

What does this MR do?

  • Adds doc/user/project/merge_requests/title_templates.md with beta status and feature flag notice
  • Adds mr_default_title_template parameter to doc/api/projects.md (attribute tables and JSON response examples)
  • Documents supported variables, examples, and configuration steps

Related

  • Depends on: API/UI MR (Part 3)
  • Part of: #16080

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