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.

docs/features/shop.md

@@ -0,0 +1,96 @@
+# Shop Page
+
+The shop page displays your product catalog with browsing and filtering options.
+
+## Overview
+
+The WooNooW shop page provides:
+
+- **Product Grid** - Visual display of products
+- **Search** - Find products by name
+- **Filters** - Category and sorting options
+- **Pagination** - Navigate through products
+
+---
+
+## Features
+
+### Product Cards
+
+Each product displays:
+- Product image
+- Product name
+- Price (with sale price if applicable)
+- Add to Cart button
+- Wishlist button (if enabled)
+
+### Search
+
+Type in the search box to filter products by name. Search is instant and updates the grid as you type.
+
+### Category Filter
+
+Filter products by category using the dropdown. Shows:
+- All Categories
+- Individual categories with product count
+
+### Sorting
+
+Sort products by:
+- Default sorting
+- Popularity
+- Average rating
+- Latest
+- Price: Low to High
+- Price: High to Low
+
+---
+
+## Customization
+
+### Grid Layout
+
+Configure the product grid in **WooNooW → Appearance**:
+
+| Device | Options |
+|--------|---------|
+| Mobile | 1-2 columns |
+| Tablet | 2-4 columns |
+| Desktop | 2-6 columns |
+
+### Product Card Style
+
+Product cards can display:
+- **Image** - Product featured image
+- **Title** - Product name
+- **Price** - Current price and sale price
+- **Rating** - Star rating (if reviews enabled)
+- **Add to Cart** - Quick add button
+
+---
+
+## Navigation
+
+### Clicking a Product
+
+Clicking a product card navigates to the full product page where customers can:
+- View all images
+- Select variations
+- Read description
+- Add to cart
+
+### Back to Shop
+
+From any product page, use the breadcrumb or browser back button to return to the shop.
+
+---
+
+## Performance
+
+### Lazy Loading
+
+Product images load as they come into view, improving initial page load time.
+
+### Infinite Scroll vs Pagination
+
+Currently uses pagination. Infinite scroll may be added in future versions.