title: ActiveAdmin, register_page and register
body: <p>Given a page defined with:</p>
  
  <pre><code>ActiveAdmin.register Car do
    index do
      column :id
      column :number_plate
    end
  end
  </code></pre>
  
  <p>I want to add a panel with some text before the items table. </p>
  
  <p>Here is what I noticed about the availability of methods for customizing admin pages.</p>
  
  <ul>
  <li><p>when i create a page with <code>register_page</code>, the methods <code>content</code>, <code>columns</code> (inside content) <code>column</code> (inside columns) are available, and the <code>index</code> method is not</p></li>
  <li><p>when i create a page with <code>register &lt;resource&gt;</code> the <code>content</code>, <code>columns</code>, <code>column</code> methods are <strong>not</strong> available while the <code>index</code> method is available </p></li>
  </ul>
  
  <p>Ideally, i want to add an <code>index</code> to pages created via <code>register_page</code>. </p>
  
  <p>If this isn't achievable, then i want to find a workaround that allows to add panels in pages created via <code>register</code>. </p>
  
  <p>I tryed with table_for method, but the result is quite different from the one obtained with index. </p>