Update config to enable docfx v3

Author: VSC-Service-Account

.openpublishing.publish.config.json

@@ -72,5 +72,8 @@
     "Pdf": {
       "template_folder": "_themes.pdf"
     }
+  },
+  "docs_build_engine": {
+    "name": "docfx_v3"
   }
-}
+}