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/development/creating-modules.md

@@ -12,7 +12,7 @@ Creating a new module requires **zero changes** to `main.py`, `registry.py`, or
 # Create module with all directories
 MODULE_NAME=mymodule
 
-mkdir -p app/modules/$MODULE_NAME/{routes/{api,pages},services,models,schemas,templates/$MODULE_NAME/store,static/store/js,locales,tasks}
+mkdir -p app/modules/$MODULE_NAME/{routes/{api,pages},services,models,schemas,templates/$MODULE_NAME/store,static/store/js,locales,tasks,docs}
 
 # Create required files
 touch app/modules/$MODULE_NAME/__init__.py
@@ -156,6 +156,10 @@ app/modules/mymodule/
 │       └── js/
 │           └── mymodule.js
 │
+├── docs/                    # Module documentation (source of truth)
+│   ├── index.md             # Module overview (REQUIRED)
+│   └── ...                  # Additional docs as needed
+│
 ├── locales/                 # Translations (auto-loaded)
 │   ├── en.json
 │   ├── de.json
@@ -538,6 +542,8 @@ python scripts/validate/validate_architecture.py
 - [ ] Create locales (en, de, fr, lu)
 - [ ] Create `config.py` if module needs environment settings (optional)
 - [ ] Create `migrations/versions/` with `__init__.py` files if module has database tables
+- [ ] Create `docs/index.md` with module overview (see [Module Documentation](module-documentation.md))
+- [ ] Create symlink: `ln -s ../../app/modules/$MODULE_NAME/docs docs/modules/$MODULE_NAME`
 - [ ] Run `python scripts/validate/validate_architecture.py`
 - [ ] Test routes are accessible
 
@@ -555,5 +561,6 @@ The framework discovers everything automatically!
 ## Related Documentation
 
 - [Module System](../architecture/module-system.md) - Architecture overview
+- [Module Documentation](module-documentation.md) - Documentation standards
 - [Menu Management](../architecture/menu-management.md) - Sidebar integration
 - [Architecture Rules](architecture-rules.md) - Validation rules