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/architecture/module-system.md

@@ -127,6 +127,11 @@ app/modules/analytics/
 ├── definition.py            # ModuleDefinition (REQUIRED for auto-discovery)
 ├── config.py                # Environment config (auto-discovered)
 ├── exceptions.py            # Module-specific exceptions
+├── docs/                    # Module documentation (source of truth)
+│   ├── index.md             # Module overview (REQUIRED)
+│   ├── data-model.md        # Entity relationships (optional)
+│   ├── api.md               # API reference (optional)
+│   └── business-logic.md    # Complex logic docs (optional)
 ├── routes/
 │   ├── __init__.py
 │   ├── api/                 # API endpoints (auto-discovered)