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/checkout.md

@@ -0,0 +1,145 @@
+# Checkout
+
+The WooNooW checkout provides a streamlined purchasing experience.
+
+## Overview
+
+The checkout process includes:
+
+1. **Cart Review** - Verify items before checkout
+2. **Customer Information** - Billing and shipping details
+3. **Payment Method** - Select how to pay
+4. **Order Confirmation** - Complete the purchase
+
+---
+
+## Checkout Flow
+
+### Step 1: Cart
+
+Before checkout, customers review their cart:
+- Product list with images
+- Quantity adjustments
+- Remove items
+- Apply coupon codes
+- See subtotal, shipping, and total
+
+### Step 2: Customer Details
+
+Customers provide:
+- **Email address**
+- **Billing information**
+  - Name
+  - Address
+  - Phone
+- **Shipping address** (if different from billing)
+
+> **Note**: Logged-in customers have their details pre-filled.
+
+### Step 3: Shipping Method
+
+If physical products are in the cart:
+- Available shipping methods are shown
+- Shipping cost is calculated
+- Customer selects preferred method
+
+### Step 4: Payment
+
+Customers choose their payment method:
+- Credit/Debit Card (Stripe, PayPal, etc.)
+- Bank Transfer
+- Cash on Delivery
+- Other configured gateways
+
+### Step 5: Place Order
+
+After reviewing everything:
+- Click "Place Order"
+- Payment is processed
+- Confirmation page is shown
+- Email receipt is sent
+
+---
+
+## Features
+
+### Guest Checkout
+
+Allow customers to checkout without creating an account.
+
+Configure in **WooCommerce → Settings → Accounts & Privacy**.
+
+### Coupon Codes
+
+Customers can apply discount codes:
+1. Enter code in the coupon field
+2. Click "Apply"
+3. Discount is reflected in total
+
+### Order Notes
+
+Optional field for customers to add special instructions.
+
+---
+
+## Payment Gateways
+
+### Supported Gateways
+
+WooNooW supports all WooCommerce payment gateways:
+
+| Gateway | Type |
+|---------|------|
+| Bank Transfer (BACS) | Manual |
+| Check Payments | Manual |
+| Cash on Delivery | Manual |
+| PayPal | Card / PayPal |
+| Stripe | Card |
+| Square | Card |
+
+### Configuring Gateways
+
+1. Go to **WooNooW → Settings → Payments**
+2. Enable desired payment methods
+3. Configure API keys and settings
+4. Test with sandbox/test mode first
+
+---
+
+## After Checkout
+
+### Order Confirmation Page
+
+Shows:
+- Order number
+- Order summary
+- Next steps
+
+### Confirmation Email
+
+Automatically sent to customer with:
+- Order details
+- Payment confirmation
+- Shipping information (if applicable)
+
+---
+
+## Troubleshooting
+
+### "Place Order" Button Not Working
+
+1. Check all required fields are filled
+2. Verify payment gateway is properly configured
+3. Check browser console for JavaScript errors
+
+### Payment Declined
+
+1. Customer should verify card details
+2. Check payment gateway dashboard for error details
+3. Ensure correct API keys are configured
+
+### Shipping Not Showing
+
+1. Verify shipping zones are configured in WooCommerce
+2. Check if products have weight/dimensions set
+3. Confirm customer's address is in a configured zone