Add documentation for merge request title templates

Marc Shawrequested to merge
docs/mr-title-template-594901 into master

What does this MR do and why?

Adds user-facing and API documentation for the merge request default title template feature (Part 4/4 of #16080).

Changes

  • New page: doc/user/project/merge_requests/title_templates.md with beta status and feature flag notice, supported variables, configuration steps, and template examples.
  • API docs: Added mr_default_title_template parameter to doc/api/projects.md attribute tables and JSON response examples.
  • Cross-references: Added links from commit templates page and project settings page.

MR acceptance checklist

  • I have evaluated the MR acceptance checklist for this MR.

References

Resolves #594901 (closed)

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