Drop datashader from packaging and install docs

[brendancol]

Reason or Problem

datashader is currently listed under the examples extra in setup.cfg. Once the source code, user-guide notebooks, and example notebooks no longer import it, the dependency declaration should go away so a fresh pip install xarray-spatial[examples] no longer pulls datashader.

This is the closing step of the wider effort to remove datashader as a dependency. xarray-spatial's own rasterize() (xrspatial/rasterize.py, available as arr.xrs.rasterize(...)) covers the vector-rasterization use cases, and matplotlib covers the visualization use cases.

Proposal

• Remove the datashader >= 0.15.0 line from setup.cfg (the examples extra).
• Make sure matplotlib (and any colormap source the notebooks now rely on) is present in the appropriate extra.
• Remove the datashader row from docs/source/getting_started/installation.rst.

Done when

• A clean environment installs xarray-spatial without datashader, and python -c "import xrspatial" succeeds.
• grep -rn datashader xrspatial/ setup.cfg docs/source/getting_started/ returns nothing.

The only datashader references left in the repo after this are the comparison benchmark and the historical-credit prose, both kept on purpose.

Blocked by

This must land last, after every issue below, otherwise [examples] users lose datashader while the notebooks still import it.

Source and benchmark:

• Remove canvas_like and datashader references from source code #3451 Remove canvas_like and datashader references from source code
• Guard the datashader import in the rasterizer benchmark #3452 Guard the datashader import in the rasterizer benchmark

User-guide notebooks:

• Migrate surface user guide off datashader #3453 surface
• Migrate proximity user guide off datashader #3454 proximity
• Migrate focal user guide off datashader #3455 focal
• Migrate zonal user guide off datashader #3456 zonal
• Migrate classification user guide off datashader #3457 classification
• Migrate multispectral user guide off datashader #3458 multispectral
• Migrate fire user guide off datashader #3459 fire
• Migrate pathfinding user guide off datashader #3460 pathfinding

Example notebooks and script:

• Migrate Pathfinding_Austin_Road_Network example off datashader #3461 Pathfinding_Austin_Road_Network
• Migrate pharmacy-deserts example off datashader #3462 pharmacy-deserts
• Migrate classification-methods example off datashader #3463 classification-methods
• Migrate animated_hillshade example script off datashader #3464 animated_hillshade.py