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/core/docs/index.md

@@ -0,0 +1,44 @@
+# Core Platform
+
+Dashboard, settings, and profile management. Required for basic operation.
+
+## Overview
+
+| Aspect | Detail |
+|--------|--------|
+| Code | `core` |
+| Classification | Core |
+| Dependencies | None |
+| Status | Active |
+
+## Features
+
+- `dashboard` — Main admin dashboard
+- `settings` — Platform and store settings management
+- `profile` — User profile management
+
+## Permissions
+
+| Permission | Description |
+|------------|-------------|
+| `dashboard.view` | View the admin dashboard |
+| `settings.view` | View settings |
+| `settings.edit` | Edit settings |
+| `settings.theme` | Manage theme settings |
+| `settings.domains` | Manage domains (owner only) |
+
+## Data Model
+
+- **AdminMenuConfig** — Stores menu configuration for admin sidebar
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `GET` | `/api/v1/admin/dashboard` | Dashboard data |
+| `GET/PATCH` | `/api/v1/admin/settings` | Settings CRUD |
+| `GET/PATCH` | `/api/v1/admin/menu-config` | Menu configuration |
+
+## Configuration
+
+No module-specific configuration.