Update pysidedocs theme to rely more on sphinx's base theme:

- Drop files that are redundant with sphinx.
- Add _templates/layout.html to add the needed customisation.
- Update theme.conf to ensure theme inheritance.
- Update pysidedocs.css to fit the theme with sphinx's basic theme.

Reviewer: Hugo Parente Lima <hugo.lima@openbossa.org>
          Renato Araújo <renato.filho@openbossa.org>

doc/_themes/pysidedocs/searchbox.html

@@ -0,0 +1,12 @@
+{%- if pagename != "search" %}
+<div id="searchbox" style="display: none">
+  <h3>{{ _('Quick search') }}</h3>
+    <form class="search" action="{{ pathto('search') }}" method="get">
+      <input type="text" name="q" id="q" size="18" />
+      <input type="submit" value="{{ _('Go') }}" id="search_button" />
+      <input type="hidden" name="check_keywords" value="yes" />
+      <input type="hidden" name="area" value="default" />
+    </form>
+</div>
+<script type="text/javascript">$('#searchbox').show(0);</script>
+{%- endif %}