Add Pocoo logo to the sidebar on the index page.

doc/_templates/indexsidebar.html

@@ -1,3 +1,6 @@
+<p class="logo"><a href="http://pocoo.org/">
+  <img src="{{ pathto("_static/pocoo.png", 1) }}" /></a></p>
+
 <h3>Download</h3>
 {% if version.endswith('(hg)') %}
 <p>This documentation is for version <b>{{ version }}</b>, which is