Migrate proximity user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/proximity.ipynb imports datashader, rasterizes points with Canvas().points(), and renders with shade/stack/dynspread.

Proposal

• Remove the datashader imports.
• Replace Canvas().points() point rasterization with arr.xrs.rasterize(...).
• Replace shade/stack/dynspread 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.