Update config to enable docfx v3

.openpublishing.publish.config.json

@@ -82,5 +82,8 @@
   "DiffFolder": [
     "dotnet/api"
   ],
-  "SplitTOC": "dotnet/api/toc.yml"
+  "SplitTOC": "dotnet/api/toc.yml",
+  "docs_build_engine": {
+    "name": "docfx_v3"
+  }
 }