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/payments/docs/index.md

@@ -0,0 +1,46 @@
+# Payment Gateways
+
+Core payment gateway integrations for Stripe, PayPal, and bank transfers. Provides payment processing, refunds, and payment method management. Required by billing module for subscription payments.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `payments` |
+| Classification | Core |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `payment_processing` — Core payment transaction processing
+- `payment_refunds` — Refund handling
+- `payment_methods` — Payment method management
+- `stripe_gateway` — Stripe integration
+- `paypal_gateway` — PayPal integration
+- `bank_transfer` — Bank transfer support
+- `transaction_history` — Transaction audit trail
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `payments.view_gateways` | View payment gateway configuration |
+| `payments.manage_gateways` | Configure payment gateways |
+| `payments.view_transactions` | View transaction history |
+
+## Data Model
+
+Payment models are managed at the framework level (Stripe webhooks, payment intents).
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `*` | `/api/v1/admin/payments/*` | Admin payment management |
+| `*` | `/api/v1/store/payments/*` | Store payment settings |
+| `POST` | `/api/v1/payments/webhooks/*` | Payment gateway webhooks |
+
+## Configuration
+
+Configured via Stripe/PayPal environment variables at the platform level.