docs: update API documentation for single POST upsert endpoint

- Replace separate POST/PUT endpoints with unified POST upsert endpoint
- Add comprehensive API usage examples with automatic ID generation
- Simplify content management to single endpoint for create/update operations
- Document element_context for backend ID generation and version control features

README.md

@@ -297,6 +297,35 @@ export INSERTR_SITE_ID="production"
 - **Links/Buttons** (`a`, `button`) → Text + URL fields
 - **Containers** (`div`, `section`) → Expand to viable children
 
+## 🔌 API Reference
+
+### **Simple Content Management**
+```bash
+# Create or update content (single endpoint for both)
+curl -X POST http://localhost:8080/api/content?site_id=demo \
+  -H "Content-Type: application/json" \
+  -d '{"value": "New content", "type": "text"}'
+
+# Get all content for a site
+curl http://localhost:8080/api/content?site_id=demo
+
+# Get specific content by ID
+curl http://localhost:8080/api/content/content-id?site_id=demo
+
+# View edit history
+curl http://localhost:8080/api/content/content-id/versions?site_id=demo
+
+# Rollback to previous version
+curl -X POST http://localhost:8080/api/content/content-id/rollback \
+  -d '{"version_id": 123}'
+```
+
+**Key Features:**
+- **Single POST endpoint** handles both create and update (upsert)
+- **Automatic ID generation** from element context if no ID provided
+- **Version control** preserves all changes with user attribution
+- **Auto-enhancement** updates static files when content changes
+
 ## 🔧 Development Workflow
 
 ### **Live Development with Hot Reload**