docs: add genindex to globaltoc

Author: blueyed

doc/en/_templates/globaltoc.html

@@ -21,3 +21,7 @@ <h3><a href="{{ pathto(master_doc) }}">{{ _('Table Of Contents') }}</a></h3>
   <hr>
   {{ toc }}
 {%- endif %}
+
+<hr>
+<a href="{{ pathto('genindex') }}">Index</a>
+<hr>