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>

app/modules/cart/docs/index.md

@@ -0,0 +1,41 @@
+# Shopping Cart
+
+Session-based shopping cart for storefronts.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `cart` |
+| Classification | Optional |
+| Dependencies | `inventory`, `catalog` |
+| Status | Active |
+
+## Features
+
+- `cart_management` — Cart creation and management
+- `cart_persistence` — Session-based cart persistence
+- `cart_item_operations` — Add/update/remove cart items
+- `shipping_calculation` — Shipping cost calculation
+- `promotion_application` — Apply promotions and discounts
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `cart.view` | View cart data |
+| `cart.manage` | Manage cart settings |
+
+## Data Model
+
+- **Cart** — Shopping cart with session tracking and store scoping
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/storefront/cart/*` | Storefront cart operations |
+
+## Configuration
+
+No module-specific configuration.