Merge pull request statsmodels#8418 from josef-pkt/notebook_treatment

DOC: add notebook for treatment effect

Author: josef-pkt

docs/source/_static/images/treatment_effect.png

@@ -128,6 +128,9 @@
   - img: ../_static/images/mediation_survival.png
     target: notebooks/generated/mediation_survival.html
     text: Mediation analysis with duration data
+  - img: ../_static/images/treatment_effect.png
+    target: notebooks/generated/treatment_effect.html
+    text: Treatment Effect Estimation
   - img: ../_static/images/copula.png
     target: notebooks/generated/copula.html
     text: Copulas