docs: add colouring points example (#20)

* Add colouring points example

New examples/colouring_points.ipynb covering render_points colour control on
the real Xenium cells transcripts (squidpy):

- continuous color= with a colorbar; cmap and norm to shape the mapping
- categorical color= with a legend; palette, groups, and na_color
- size and alpha
- uses the matplotlib backend for per-point colour (datashader/density noted
  for scale, cross-linked to the density and performance notebooks)

Executed against spatialdata-plot v0.4.1; committed with outputs. Adds the
examples gallery card + toctree + thumbnail. Closes P2.1 of the doc-coverage
analysis.

* docs(colouring_points): add dataset blurb + transcripts columns, na_color-hides-unmatched rule, concrete 10k datashader threshold

Author: timtreis

_static/img/colouring_points.png

@@ -24,6 +24,16 @@ Draw regions of interest directly on a `spatialdata-plot` canvas with
 `sdata.pl.annotate(...)` and persist them as a `ShapesModel` element.
 :::
 
+:::{grid-item-card} Colouring points
+:link: /notebooks/examples/colouring_points
+:link-type: doc
+:img-top: /notebooks/_static/img/colouring_points.png
+
+Colour a point element by continuous (`cmap`, `norm`, colorbar) or
+categorical (`palette`, `groups`, `na_color`, legend) columns, and tune
+`size`/`alpha`, on real Xenium transcripts.
+:::
+
 ::::
 <!-- gallery-cards-end -->
 
@@ -33,4 +43,5 @@ Draw regions of interest directly on a `spatialdata-plot` canvas with
 
 visium_mouse_brain
 interactive_annotate
+colouring_points
 ```