Revive Visium card under Examples in gallery

PR #657 added the card originally; the placeholder note slipped back
in during a later rebase. Restores the card and ships the refreshed
thumbnail from the updated notebook (publication-style figure
colouring spots by Mbp expression on the H&E tissue).

Author: timtreis

docs/_static/gallery/visium_mouse_brain.png

@@ -38,11 +38,21 @@ perceptually well-spaced or colourblind-safe palettes with
 
 ## Examples
 
-```{note}
-No focused examples yet — contributions welcome! See
-[CONTRIBUTING](https://github.com/scverse/spatialdata-plot-notebooks/blob/main/CONTRIBUTING.md)
-in the notebooks repo for how to add one.
-```
+Worked examples on real datasets you'd actually analyse.
+
+::::{grid} 1 2 2 2
+:gutter: 3
+
+:::{grid-item-card} Visium mouse brain
+:link: notebooks/examples/visium_mouse_brain
+:link-type: doc
+:img-top: _static/gallery/visium_mouse_brain.png
+
+Render H&E tissue, overlay spots, colour by gene expression and by
+cluster, and finish with a publication-style figure.
+:::
+
+::::
 
 ```{toctree}
 :hidden: