This commit adds the complete admin workflow for managing homepage curated route subgroups:
- Add `RouteSection` type and extend `SiteConfig`/`SiteModule` to support the new module
- Create all required admin components: panel, editors, product picker, and utility functions
- Update admin utilities and API types to handle routeSections CRUD operations
- Update documentation to include the new routeSections API contract
- Add supporting CSS styles for the new UI elements
