docs: fix template to support search result preview

docs/_templates/layout.html

@@ -122,7 +122,7 @@
             </ol>
           </div>
           {% endblock %}
-          <div class="document clearer body">
+          <div class="document clearer body" role="main">
             {% block body %} {% endblock %}
           </div>
           {%- block bottom_rel_links %}