Update config to enable docfx v3

Author: VSC-Service-Account

.openpublishing.publish.config.json

@@ -118,5 +118,8 @@
   "need_generate_pdf": false,
   "need_generate_intellisense": false,
   "enable_branch_build_custom_validation": true,
-  "enable_pull_request_custom_validation": true
-}
+  "enable_pull_request_custom_validation": true,
+  "docs_build_engine": {
+    "name": "docfx_v3"
+  }
+}