Migrate surface user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/surface.ipynb imports datashader and uses Canvas to set up the terrain grid plus shade/stack and the Elevation colormap for rendering. xarray-spatial now provides its own rasterization, and matplotlib covers the rendering.

Proposal

• Remove the datashader imports.
• Replace any Canvas-based grid setup with xarray-spatial's own functions (e.g. generate_terrain with a plain coordinate setup).
• Replace shade/stack and datashader colormaps with matplotlib (imshow, a terrain-style 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.