add repo migration guide

[necoline]

Summary

• Added a migration guide page (migration-guide.mdx) to the Repositories section covering vocabulary mapping, workflow transition, devcontainer.json setup, and key differences between
  CodeSandbox Repos and GitHub Codespaces
• Added deprecation warning callouts linking to the migration guide on all repository doc pages: overview, PR reviews, open source, collaborate & share, and repo import
• Updated _meta.json to surface the migration guide as the first item in the Repositories sidebar

Context

CodeSandbox Repositories will stop accepting new imports on April 1, 2026 and fully shut down on July 1, 2026. This PR ensures users see the deprecation notice wherever they encounter
repository docs and have a clear path to migrate.

Test plan

• Verify the migration guide renders correctly at /learn/repositories/migration-guide
• Confirm the warning callout appears on all repository pages
• Check that the sidebar nav shows "Migration Guide" as the first item under Repositories
• Verify all internal links resolve correctly

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Project	Deployment	Actions	Updated (UTC)
projects-docs	Ready	Preview, Comment	Mar 13, 2026 0:50am

[christianalfoni]

✅ Approved

Summary

This PR adds a comprehensive migration guide and deprecation notices for CodeSandbox Repositories, which will stop accepting new imports on April 1, 2026 and fully shut down on July 1, 2026.

What's Included

1. New Migration Guide (migration-guide.mdx) - A well-structured, comprehensive guide covering:

   • Vocabulary mapping between CodeSandbox and GitHub Codespaces concepts
   • Step-by-step instructions for setting up Prebuilds and devcontainer.json
   • Key differences around persistence, secrets, and collaboration
   • Clear timeline for the deprecation

2. Deprecation Warning Callouts - Added consistent warning callouts to all repository documentation pages:

   • overview.mdx
   • review.mdx
   • open-source.mdx
   • collaborate-share.mdx
   • getting-started/repo-import.mdx

3. Sidebar Navigation Update - Added "Migration Guide" as the first item in the Repositories sidebar via _meta.json

Quality Assessment

✅ Clear and actionable - The migration guide provides specific steps users can follow
✅ Comprehensive coverage - All relevant repository pages have the deprecation notice
✅ Consistent messaging - Same dates and link to migration guide across all pages
✅ Good timing - Plenty of advance notice before the April 2026 deadline
✅ Helpful context - Vocabulary mapping table helps users understand concept translations

Minor Observations (Non-blocking)

• The deprecation callout text is duplicated verbatim across 5 files. For documentation, this is acceptable and sometimes preferred for self-contained pages, though a shared component could be considered for future maintenance.
• The Callout emoji usage varies slightly between files (some use emoji prop, others don't), but this doesn't affect functionality.

Overall, this is a well-executed documentation update that will help users navigate the transition smoothly.