feat: Add in-app documentation system

Phase 1: Core Documentation
- Created docs/ folder with 8 markdown documentation files
- Getting Started, Installation, Troubleshooting, FAQ
- Configuration docs (Appearance, SPA Mode)
- Feature docs (Shop, Checkout)
- PHP registry with filter hook for addon extensibility

Phase 2: Documentation Viewer
- DocsController.php with REST API endpoints
- GET /woonoow/v1/docs - List all docs (with addon hook)
- GET /woonoow/v1/docs/{slug} - Get document content
- Admin SPA /help route with sidebar navigation
- Markdown rendering with react-markdown
- Added Help & Docs to More page for mobile access

Filter Hook: woonoow_docs_registry
Addons can register their own documentation sections.

admin-spa/package.json

@@ -49,8 +49,10 @@
         "qrcode": "^1.5.4",
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
+        "react-markdown": "^10.1.0",
         "react-router-dom": "^7.9.4",
         "recharts": "^3.3.0",
+        "remark-gfm": "^4.0.1",
         "sonner": "^2.0.7",
         "tailwind-merge": "^3.3.1",
         "vaul": "^1.1.2",