Add comprehensive blog demo showcasing advanced content management features

- Implement complete mushroom foraging blog with chanterelles article
- Add rich demonstration of .insertr and .insertr-add functionality
- Include comprehensive documentation for future .insertr-content vision
- Update project styling and configuration to support blog demo
- Enhance engine and API handlers for improved content management

insertr.yaml

@@ -12,6 +12,10 @@ database:
 server:
   port: 8080            # HTTP API server port
   sites:                # Registered sites for file-based enhancement
+    - site_id: "blog"
+      path: "./demos/blog_enhanced"
+      source_path: "./demos/blog/"
+      auto_enhance: true
     - site_id: "default"
       path: "./demos/default_enhanced"
       source_path: "./demos/default"