Merge pull request MicrosoftDocs#6604 from MicrosoftDocs/docs-build-v3

Update config to enable docfx v3

Author: chrisda

.openpublishing.publish.config.json

@@ -253,5 +253,8 @@
       "target_framework": "net45",
       "version": "latest"
     }
-  ]
+  ],
+  "docs_build_engine": {
+    "name": "docfx_v3"
+  }
 }