Merge pull request statsmodels#8412 from josef-pkt/notebook_rates

DOC: add notebook for stats poisson rates

Author: josef-pkt

docs/source/_static/images/stats_poisson.png

@@ -119,6 +119,9 @@
   - img: ../_static/images/anova.png
     target: notebooks/generated/interactions_anova.html
     text: ANOVA
+  - img: ../_static/images/stats_poisson.png
+    target: notebooks/generated/stats_poisson.html
+    text: Poisson rates
   - img: ../_static/images/rank_compare.png
     target: notebooks/generated/stats_rankcompare.html
     text: Rank Comparison