Add hybrid editing system with markdown collections

Implement markdown collection support alongside existing element-level editing. Users can now choose between individual element editing (headlines, buttons) and rich markdown editing (stories, bios). Includes responsive overlay forms, smart field detection, and comprehensive documentation updates.

Features:
- Markdown collections with data-field-type="markdown"
- Responsive overlay forms that adapt to element width
- Element-level editing for granular control
- Updated documentation and integration guide
- Fixed marked.js renderer configuration issues
- Enhanced about.html to demonstrate both approaches

demo-site/index.html

@@ -85,6 +85,7 @@
     </footer>
 
     <!-- Insertr JavaScript Library -->
+    <script src="https://cdn.jsdelivr.net/npm/marked@16.2.1/lib/marked.umd.js"></script>
     <script src="insertr/insertr.js"></script>
 </body>
 </html>