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.

includes/Api/DocsController.php

@@ -0,0 +1,129 @@
+<?php
+/**
+ * Documentation API Controller
+ * 
+ * Serves documentation content to the Admin SPA.
+ */
+
+namespace WooNooW\Api;
+
+use WP_REST_Controller;
+use WP_REST_Response;
+use WP_REST_Request;
+use WP_Error;
+
+class DocsController extends WP_REST_Controller {
+    
+    /**
+     * Namespace for REST routes
+     */
+    protected $namespace = 'woonoow/v1';
+    
+    /**
+     * Base route
+     */
+    protected $rest_base = 'docs';
+    
+    /**
+     * Register routes
+     */
+    public function register_routes() {
+        // GET /woonoow/v1/docs - List all documentation
+        register_rest_route($this->namespace, '/' . $this->rest_base, [
+            [
+                'methods'             => 'GET',
+                'callback'            => [$this, 'get_docs_registry'],
+                'permission_callback' => [$this, 'check_permissions'],
+            ],
+        ]);
+        
+        // GET /woonoow/v1/docs/{slug} - Get single document
+        register_rest_route($this->namespace, '/' . $this->rest_base . '/(?P<slug>.+)', [
+            [
+                'methods'             => 'GET',
+                'callback'            => [$this, 'get_doc'],
+                'permission_callback' => [$this, 'check_permissions'],
+                'args'                => [
+                    'slug' => [
+                        'required'          => true,
+                        'type'              => 'string',
+                        'sanitize_callback' => 'sanitize_text_field',
+                    ],
+                ],
+            ],
+        ]);
+    }
+    
+    /**
+     * Check permissions - any logged in admin user
+     */
+    public function check_permissions($request) {
+        return current_user_can('manage_options');
+    }
+    
+    /**
+     * Get documentation registry
+     * 
+     * @return WP_REST_Response
+     */
+    public function get_docs_registry($request) {
+        require_once WOONOOW_PLUGIN_DIR . 'docs/_registry.php';
+        
+        $registry = \WooNooW\Docs\get_docs_registry();
+        
+        // Transform to frontend format (without file paths)
+        $result = [];
+        foreach ($registry as $section_key => $section) {
+            $items = [];
+            foreach ($section['items'] as $item) {
+                $items[] = [
+                    'slug'  => $item['slug'],
+                    'title' => $item['title'],
+                ];
+            }
+            
+            $result[] = [
+                'key'   => $section_key,
+                'label' => $section['label'],
+                'icon'  => $section['icon'] ?? 'file-text',
+                'items' => $items,
+            ];
+        }
+        
+        return new WP_REST_Response([
+            'success'  => true,
+            'sections' => $result,
+        ], 200);
+    }
+    
+    /**
+     * Get single document content
+     * 
+     * @param WP_REST_Request $request
+     * @return WP_REST_Response|WP_Error
+     */
+    public function get_doc($request) {
+        $slug = $request->get_param('slug');
+        
+        require_once WOONOOW_PLUGIN_DIR . 'docs/_registry.php';
+        
+        $doc = \WooNooW\Docs\get_doc_by_slug($slug);
+        
+        if (!$doc) {
+            return new WP_Error(
+                'doc_not_found',
+                'Documentation not found',
+                ['status' => 404]
+            );
+        }
+        
+        return new WP_REST_Response([
+            'success' => true,
+            'doc'     => [
+                'slug'    => $doc['slug'],
+                'title'   => $doc['title'],
+                'content' => $doc['content'],
+            ],
+        ], 200);
+    }
+}