Add vignettes that provide an answer to an interesting question

E.g., https://github.com/idem-lab/vector_atlas_training_2024/blob/main/R/spatial%20smoothing%20models.R

Start by reviewing the linked spatial smoothing models.R example and the repository's existing vignette structure. Identify an interesting question that fits greta.gam and define the expected answer before adding a vignette; done means the vignette provides a reproducible answer.

Written by the indexing model from the issue text.