Update config to enable docfx v3

Author: VSC-Service-Account

.openpublishing.publish.config.json

@@ -39,7 +39,7 @@
         "SourceXmlFolder": "docs/Mono.Android/en",
         "OutputYamlFolder": "docs/Mono.Android/en/api",
         "Flatten": true,
-        "id": "XamarinAndroidAPI",        
+        "id": "XamarinAndroidAPI",
         "SDPMode": "true"
       },
       "SplitTOC": [
@@ -90,5 +90,8 @@
       "target_framework": "net45",
       "version": "latest"
     }
-  ]
+  ],
+  "docs_build_engine": {
+    "name": "docfx_v3"
+  }
 }