plot_cap example for docs (#670)

* plot_cap draft outline for docs example

* intro. to GLMs and Negative Binomial model

* added logistic regression and other model params. demo

* add plot_cap.ipynb to .rst file

* plot other model params. and formatting

* improve explanations, explain datasets, and add watermark

* changed quantities of interest and model components explanation

Author: GStechschulte

docs/examples.rst

@@ -26,3 +26,4 @@ Examples
   notebooks/distributional_models
   notebooks/hsgp_1d
   notebooks/hsgp_2d
+  notebooks/plot_cap