title: Sort index alphabetically on first, second, third characters
body: <p>I have a df which looks like this:</p>
  
  <pre><code>df = pd.DataFrame({'val': [0, 0, 0, 1, 0, 0, 0]}, 
                    index=['13th str', '3SAT', 'ARD', 
                           'ARD Dritte', 'AXNAction', 'Animal', 'bb'])
              val
  13th str      0
  3SAT          0
  ARD           0
  ARD Dritte    1
  AXNAction     0
  Animal        0
  bb            0
  </code></pre>
  
  <p>I would like to sort it to look like this, </p>
  
  <pre><code>            val
  13th str      0
  3SAT          0
  Animal        0
  ARD           0
  ARD Dritte    1
  AXNAction     0
  bb            0
  </code></pre>
  
  <p>note - 'Animal' has shifted places.</p>
  
  <p>If all the first letters are the same then look at the next character and so on. </p>
  
  <p>Here is what I have tried which has not worked:</p>
  
  <pre><code>df.sort()
  df = df.sort_index()
  df = df.index.sort_values() #gives an 'Index' object has no attribute 'sort_values' error
  </code></pre>