Drop datashader from packaging and install docs

Wait until the listed source, benchmark, notebook, and example issues are complete, then inspect the examples extra in setup.cfg and the dependency table in docs/source/getting_started/installation.rst. Remove the remaining datashader entries, ensure the stated plotting dependency is covered, and verify a clean examples install, the xrspatial import, and the specified grep command.

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:

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

User-guide notebooks:

• #3453 surface
• #3454 proximity
• #3455 focal
• #3456 zonal
• #3457 classification
• #3458 multispectral
• #3459 fire
• #3460 pathfinding

Example notebooks and script:

• #3461 Pathfinding_Austin_Road_Network
• #3462 pharmacy-deserts
• #3463 classification-methods
• #3464 animated_hillshade.py