Add section on PostgreSQL to the guides index [ci skip]

guides/source/documents.yaml

@@ -155,6 +155,10 @@
       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: Active Record and PostgreSQL
+      url: active_record_postgresql.html
+      description: This guide covers PostgreSQL specific usage of Active Record.
 
 -
   name: Extending Rails