Update config to enable docfx v3

Author: VSC-Service-Account

.openpublishing.publish.config.json

@@ -276,5 +276,8 @@
       "target_framework": "net45",
       "version": "latest"
     }
-  ]
-}
+  ],
+  "docs_build_engine": {
+    "name": "docfx_v3"
+  }
+}