docs: add point density maps example

examples/index.md

@@ -34,6 +34,16 @@ categorical (`palette`, `groups`, `na_color`, legend) columns, and tune
 `size`/`alpha`, on real Xenium transcripts.
 :::
 
+:::{grid-item-card} Point density maps
+:link: /notebooks/examples/point_density
+:link-type: doc
+:img-top: /notebooks/_static/img/point_density.png
+
+Turn tens of millions of Xenium transcripts into a readable density
+heatmap with `render_points(density=True)`, reshape it with `density_how`,
+and draw a separate density per gene.
+:::
+
 ::::
 <!-- gallery-cards-end -->
 
@@ -44,4 +54,5 @@ categorical (`palette`, `groups`, `na_color`, legend) columns, and tune
 visium_mouse_brain
 interactive_annotate
 colouring_points
+point_density
 ```