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/checkout/docs/index.md

@@ -0,0 +1,41 @@
+# Checkout
+
+Checkout and order creation for storefronts.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `checkout` |
+| Classification | Optional |
+| Dependencies | `cart`, `orders`, `payments`, `customers` |
+| Status | Active |
+
+## Features
+
+- `checkout_flow` — Multi-step checkout process
+- `order_creation` — Cart-to-order conversion
+- `payment_processing` — Payment integration during checkout
+- `checkout_validation` — Address and cart validation
+- `guest_checkout` — Checkout without customer account
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `checkout.view_settings` | View checkout settings |
+| `checkout.manage_settings` | Manage checkout configuration |
+
+## Data Model
+
+Checkout is a stateless flow that creates orders — no dedicated models.
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/storefront/checkout/*` | Storefront checkout flow |
+
+## Configuration
+
+No module-specific configuration.