docs: Add REST API docs for version control endpoints

[willeastcott]

Fixes playcanvas/editor#162
Partially addresses playcanvas/editor#498

Summary

• Add documentation for 4 version control REST API endpoints: create branch (POST /branches), list checkpoints (GET /branches/:id/checkpoints), get checkpoint (GET /checkpoints/:id), and create checkpoint (POST /checkpoints)
• Add Japanese translations for all 4 new pages
• Update sidebar to include new entries alphabetically in the REST API section

Test plan

• Verify pages render correctly with npm run start
• Verify Japanese translations render with npm run start -- --locale ja
• Confirm sidebar shows new entries in alphabetical order
• Verify all internal links (rate limiting anchors) resolve correctly

Made with Cursor

[Copilot]

Pull request overview

This PR adds REST API documentation for four version control endpoints that enable programmatic management of branches and checkpoints in PlayCanvas projects. The documentation includes both English originals and Japanese translations, following the established documentation patterns.

Changes:

• Add documentation for branch creation (POST /branches) and checkpoint management (GET/POST /checkpoints, GET /branches/:id/checkpoints)
• Include Japanese translations for all new API documentation pages
• Update sidebar navigation to include new entries in alphabetical order

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sidebars.js	Adds 4 new API documentation entries in alphabetical order within the REST API section
docs/user-manual/api/branch-create.md	Documents POST endpoint for creating new branches from existing branches/checkpoints
docs/user-manual/api/checkpoint-create.md	Documents POST endpoint for creating checkpoints that capture branch state
docs/user-manual/api/checkpoint-get.md	Documents GET endpoint for retrieving a checkpoint by ID
docs/user-manual/api/checkpoint-list.md	Documents GET endpoint for listing checkpoints for a branch with pagination
i18n/ja/docusaurus-plugin-content-docs/current/user-manual/api/branch-create.md	Japanese translation of branch creation documentation
i18n/ja/docusaurus-plugin-content-docs/current/user-manual/api/checkpoint-create.md	Japanese translation of checkpoint creation documentation
i18n/ja/docusaurus-plugin-content-docs/current/user-manual/api/checkpoint-get.md	Japanese translation of checkpoint retrieval documentation
i18n/ja/docusaurus-plugin-content-docs/current/user-manual/api/checkpoint-list.md	Japanese translation of checkpoint listing documentation

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