Move "Advanced Active Record" guides to separate section [ci-skip]

The "Digging Deeper" section is currently very large. We can move the
advanced Active Record guides to their own section.

Author: p8

guides/source/documents.yaml

@@ -235,6 +235,10 @@
       name: Using Rails for API-only Applications
       url: api_app.html
       description: This guide explains how to effectively use Rails to develop a JSON API application.
+
+-
+  name: Advanced Active Record
+  documents:
     -
       name: Active Record and PostgreSQL
       work_in_progress: true