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

Summary

Part 4 of 4 for customizable MR default title templates (#16080).

Adds user-facing and API documentation for the feature.

Scope

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

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