Migrate pharmacy-deserts example off datashader

[brendancol]

Reason or Problem

examples/pharmacy-deserts.ipynb imports datashader and uses Canvas().points()/Canvas().polygons() with ds.mean/ds.max aggregation, plus shade/stack and the inferno colormap for rendering.

Proposal

• Remove the datashader imports.
• Replace the Canvas point/polygon rasterization with arr.xrs.rasterize(...), mapping the ds.mean/ds.max aggregation onto the column= and merge= parameters.
• Replace shade/stack and the inferno colormap with matplotlib (matplotlib has an inferno colormap).
• Update any prose that mentions datashader.
• Re-execute the notebook so outputs 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.