Mark API guide as work in progress

The documentation team didn't reviewed it yet so it can't be published.

guides/source/documents.yaml

@@ -137,6 +137,7 @@
       description: This guide explains how to profile your Rails applications to improve performance.
     -
       name: Using Rails for API-only Applications
+      work_in_progress: true
       url: api_app.html
       description: This guide explains how to effectively use Rails to develop a JSON API application.