Migrate classification user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/classification.ipynb imports datashader, uses Canvas to aggregate values into a raster for the classification examples, and renders with shade/stack.

Proposal

• Remove the datashader imports.
• Replace the Canvas aggregation with arr.xrs.rasterize(...).
• Replace shade/stack and datashader colormaps with matplotlib.
• 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.