Add index and modindex to sidebar

- https://stackoverflow.com/questions/25243482/how-to-add-sphinx-generated-index-to-the-sidebar-when-using-read-the-docs-theme

docs/templates/layout.html

@@ -0,0 +1,7 @@
+{% extends "!layout.html" %}
+
+  {% block menu %}
+    {{ super() }}
+    <a href="genindex.html">Index</a>
+    <a href="py-modindex.html">Module Index</a>
+  {% endblock %}