Add MMM Quickstart Guide (#1972)

* init

* rerun

* add qs

* add qs to readme

* more resources

* improvements

* better description

* prior reference

* minor improvements

Author: juanitorduz

README.md

@@ -131,7 +131,7 @@ Once the model is fitted, we can further optimize our budget allocation as we ar
     <img src="docs/source/_static/mmm_plot_plot_channel_contributions_grid.png" width="80%" />
 </center>
 
-- Explore a hands-on [simulated example](https://pymc-marketing.readthedocs.io/en/stable/notebooks/mmm/mmm_example.html) for more insights into MMM with PyMC-Marketing.
+- Explore a hands-on our [quickstart guide](https://pymc-marketing.readthedocs.io/en/stable/notebooks/mmm/mmm_quickstart.html) and more complete [simulated example](https://pymc-marketing.readthedocs.io/en/stable/notebooks/mmm/mmm_example.html) for more insights into MMM with PyMC-Marketing.
 - Get started with a complete end-to-end analysis: from model specification to budget allocation. See the [guide notebook](https://www.pymc-marketing.io/en/stable/notebooks/mmm/mmm_case_study.html).
 
 ### Essential Reading for Marketing Mix Modeling (MMM)

docs/source/gallery/gallery.md

@@ -16,7 +16,12 @@ Welcome to the PyMC-Marketing example gallery! This gallery provides visual navi
 ::::{grid} 1 2 3 3
 :gutter: 3
 
-:::{grid-item-card} MMM Example: Introduction Notebook
+:::{grid-item-card} MMM Quickstart Guide
+:img-top: ../gallery/images/mmm_quickstart.png
+:link: ../notebooks/mmm/mmm_quickstart.html
+:::
+
+:::{grid-item-card} MMM Example: Extensive Introduction Notebook
 :img-top: ../gallery/images/mmm_example.png
 :link: ../notebooks/mmm/mmm_example.html
 :::