add edit-this-page functionality

https://buildwithfern.com/learn/docs/getting-started/global-configuration#instances-configuration

Author: humford

fern/docs.yml

@@ -1,7 +1,12 @@
 instances:
   - url: https://vapi.docs.buildwithfern.com
     custom-domain: docs.vapi.ai
-  
+    edit-this-page: 
+      github: 
+        owner: VapiAI
+        repo: docs
+        branch: main
+
 announcement: 
   message: "🚀 Vapi now provides server SDKs! Check out the [supported languages](/server-sdks)."