Notebook: How to model upper funnel with causal reasoning and PyMC (#1971)

* Notebook: How to model upper funnel with causal reasoning and pymc marketing?

* minor grammar pass.

* Daniel feedback

* Daniel feedback

* Update mmm_upper_funnel_causal_approach.ipynb

* Update mmm_upper_funnel_causal_approach.ipynb

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Co-authored-by: daniel-saunders-phil <dsaunders406@gmail.com>

Author: cetagostini

docs/source/gallery/gallery.md

@@ -152,8 +152,13 @@ Welcome to the PyMC-Marketing example gallery! This gallery provides visual navi
 :link: ../notebooks/mmm/mmm_counterfactuals.html
 :::
 
+::::{grid-item-card} Upper-funnel causal approach
+:img-top: ../gallery/images/mmm_causal_identification.png
+:link: ../notebooks/mmm/mmm_upper_funnel_causal_approach.html
 ::::
 
+:::::
+
 ### Case Studies
 
 ::::{grid} 1 2 3 3