docs: reorganize migration docs into dedicated subfolder

- Create docs/development/migration/ directory
- Move database-migrations.md to migration subfolder
- Move svc-006-migration-plan.md to migration subfolder
- Update all cross-references in:
  - mkdocs.yml nav configuration
  - docs/index.md
  - docs/architecture/overview.md
  - docs/backend/overview.md
  - docs/development/contributing.md
  - docs/development/troubleshooting.md
  - docs/getting-started/database-setup.md

This separates migration plans from core documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/getting-started/database-setup.md

@@ -331,5 +331,5 @@ python scripts/create_landing_page.py
 
 - [Shop Setup Guide](../guides/shop-setup.md) - Configure vendor storefronts
 - [Landing Pages Guide](../features/vendor-landing-pages.md) - Customize landing pages
-- [Database Migrations Guide](../development/database-migrations.md) - Advanced migration workflows
+- [Database Migrations Guide](../development/migration/database-migrations.md) - Advanced migration workflows
 - [API Documentation](../api/index.md) - Start building features