Add div.document to the theme.

This refs: https://github.com/rtfd/readthedocs.org/issues/794
All of the standard Sphinx themes include this,
and RTD uses it internally.

sphinx_rtd_theme/layout.html

@@ -113,7 +113,7 @@
       <div class="wy-nav-content">
         <div class="rst-content">
           {% include "breadcrumbs.html" %}
-          <div role="main">
+          <div role="main" class="document">
             {% block body %}{% endblock %}
           </div>
           {% include "footer.html" %}