title: Generate internal and external docs from single source using Sphinx
body: <p>I'm trying to find information on Sphinx's ability to tag certain bits of documentation as 'internal' and build it like <code>sphinx-build -b -include internal source output</code> where the <code>-include</code> flag would contain a series of tags to include in the output. </p>
  
  <p>That would allow me to generate 2 versions (or more) of my docs while keeping it single-source.</p>
  
  <p>Example:</p>
  
  <pre><code>.. audience::internal
  
  Page title
  ============
  
  This whole page will be available only in the internal docs.
  </code></pre>
  
  <p>or </p>
  
  <pre><code>Page title
  ============
  
  This whole page will be available in the internal and external docs. 
  
  .. audience::internal
  
  That p will only be available in the internal docs.
  
  This p will be available in the internal and external docs.
  </code></pre>
  
  <p>Is there such a mechanism? If not, would that be difficult to create?</p>