Migrate focal user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/focal.ipynb imports datashader, uses Canvas to aggregate values into a raster, and renders with shade/stack and the Set1 colormap.

Proposal

• Remove the datashader imports.
• Replace the Canvas aggregation with arr.xrs.rasterize(...).
• Replace shade/stack and the Set1 colormap with matplotlib (matplotlib has a Set1 colormap).
• Update the prose cells that describe the datashader workflow.
• Re-execute the notebook so outputs and the preview image regenerate cleanly.

Done when

• The notebook runs top to bottom with no datashader import and produces the expected plots.

Part of removing datashader as a dependency; final packaging removal is tracked in #3450.