Add Visium breast-cancer tutorial

Synthesises the example progression curated by @asarigun in
scverse/spatialdata-plot#590 (H&E + spots, gene-expression overlay,
outline styling) into a single end-to-end tutorial. Inlines the
load_visium_breast_cancer helper so the notebook is self-contained when
downloaded.

Uses scanpy.datasets.visium_sge for the data; pooch caches the ~100MB
download across runs. Pre-executed and committed with outputs.

Author: timtreis

tutorials/index.md

@@ -8,4 +8,5 @@ spatial elements, and styling a publication-ready figure.
 :maxdepth: 1
 
 getting_started
+visium_breast_cancer
 ```