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/src/routes/More/index.tsx

@@ -1,6 +1,6 @@
 import React, { useEffect } from 'react';
 import { useNavigate, Link } from 'react-router-dom';
-import { Tag, Settings as SettingsIcon, Palette, ChevronRight, Minimize2, LogOut, Sun, Moon, Monitor, ExternalLink, Mail, Megaphone } from 'lucide-react';
+import { Tag, Settings as SettingsIcon, Palette, ChevronRight, Minimize2, LogOut, Sun, Moon, Monitor, ExternalLink, Mail, Megaphone, HelpCircle } from 'lucide-react';
 import { __ } from '@/lib/i18n';
 import { usePageHeader } from '@/contexts/PageHeaderContext';
 import { useApp } from '@/contexts/AppContext';
@@ -32,6 +32,12 @@ const menuItems: MenuItem[] = [
     label: __('Settings'),
     description: __('Configure your store settings'),
     to: '/settings'
+  },
+  {
+    icon: <HelpCircle className="w-5 h-5" />,
+    label: __('Help & Docs'),
+    description: __('Documentation and guides'),
+    to: '/help'
   }
 ];