Migrate multispectral user guide off datashader

[brendancol]

Reason or Problem

docs/source/user_guide/multispectral.ipynb imports datashader, uses Canvas().raster() to resample bands, and renders with the transfer functions and the Images helper.

Canvas().raster() is raster resampling, not vector rasterization, so arr.xrs.rasterize does not replace it directly.

Proposal

• Remove the datashader imports.
• Replace the Canvas().raster() resampling step with a plain xarray/numpy resample, or drop it if it was only cosmetic.
• Replace the transfer functions, Images, 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.