Add a why-xarray-spatial page; retire raster_huh

[brendancol]

Refs #3249 (part 5 of 6).

raster_huh.rst was two paragraphs, and the GDAL note lost its home when usage.rst was replaced (#3258). This page gives both a permanent spot and adds the positioning the docs never had:

• "What is a raster?": the raster_huh paragraphs, unchanged.
• "What xarray-spatial is": the 150+ function summary and the four-backend dispatch story, with pointers to the README feature matrix and the stability policy for per-backend tiers.
• "No GDAL, no GEOS": the GDAL note from the old usage page, updated to present tense. The old text said compute was pure Python but was written when I/O still went through other libraries; it now matches the README's current claim that raster I/O, reprojection, and codecs are also GDAL-free.
• "How it fits the ecosystem": any 2D DataArray works as input (rioxarray output included), plus the Datashader origin sentence.

Toctree order becomes why_xarray_spatial, installation, quickstart: motivation, then setup, then doing.

Test plan:

• sphinx-build -b html clean, pre-existing warnings only
• no remaining references to raster_huh anywhere in docs/

[brendancol]

PR Review: Add a why-xarray-spatial page; retire raster_huh

Blockers

None.

Suggestions

• why_xarray_spatial.rst ("What xarray-spatial is"): "Every function takes an xr.DataArray and returns an xr.DataArray" overclaims. polygonize returns vector geometries, rasterize consumes them, and the pathfinding functions take point arguments. Describe it as the core pattern rather than a universal rule.

Nits

None.

What looks good

• The raster_huh paragraphs move unchanged, and the GDAL note keeps its original tone while the final paragraph now matches the README's current no-GDAL/no-GEOS claim instead of the old "algorithms only" wording.
• Toctree lands at the intended final order (motivation, setup, doing) and no references to raster_huh remain.
• Backend story is accurate: dispatch is by input array type, and the tier caveat links the stability policy instead of restating it.

Checklist

• Code-level items: not applicable (docs-only)
• Docs build: clean, pre-existing warnings only