Migrate zonal user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/zonal.ipynb imports datashader, rasterizes line segments with Canvas().line(), and renders with shade/stack and the Elevation colormap.

Proposal

• Remove the datashader imports.
• Replace Canvas().line() line rasterization with arr.xrs.rasterize(...).
• Replace shade/stack and the Elevation colormap with matplotlib.
• Update the prose cells, including the line that describes zones as "any region computable as a Datashader aggregate".
• 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.