docs: migrate module documentation to single source of truth

Move 39 documentation files from top-level docs/ into each module's
docs/ folder, accessible via symlinks from docs/modules/. Create
data-model.md files for 10 modules with full schema documentation.
Replace originals with redirect stubs. Remove empty guide stubs.

Modules migrated: tenancy, billing, loyalty, marketplace, orders,
messaging, cms, catalog, inventory, hosting, prospecting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/index.md

@@ -194,9 +194,6 @@ platform.com/admin                 → Admin Interface
 ### 📚 User Guides
 
 - [**User Management**](guides/user-management.md) - Managing users and roles
-- [**Product Management**](guides/product-management.md) - Working with products
-- [**Shop Setup**](guides/shop-setup.md) - Configuring store shops
-- [**CSV Import**](guides/csv-import.md) - Bulk product import
 - [**Marketplace Integration**](guides/marketplace-integration.md) - External marketplace setup
 
 ### 🧪 Testing