add sensitivity guide

Author: SvenKlaassen

doc/examples/index.rst

@@ -38,7 +38,7 @@ These are case studies with the Python package :ref:`DoubleML <doubleml_package>
     py_double_ml_sensitivity.ipynb
 
 Sandbox
------
+----------
 
 These are examples which are work-in-progress and/or not yet fully documented.