Add polygon clipping function (#1144)

[brendancol]

Summary

• clip_polygon() masks raster pixels outside a polygon to nodata (NaN by default), with optional bounding-box crop
• All four backends: numpy, dask+numpy, cupy, dask+cupy
• Accepts shapely Polygon/MultiPolygon, geometry lists, GeoDataFrame/GeoSeries, or raw [(x, y), ...] coords
• Uses rasterize() internally for the polygon-to-mask step

What changed

• xrspatial/polygon_clip.py — new module with clip_polygon()
• 20 tests across all backends, edge cases (NaN input, single-cell raster, non-overlapping geometry), GeoDataFrame/GeoSeries
• .xrs.clip_polygon() accessor on DataArrays
• API entry in docs/source/reference/zonal.rst
• User guide notebook: examples/user_guide/50_Polygon_Clipping.ipynb
• README feature matrix row under Zonal

Test plan

• pytest xrspatial/tests/test_polygon_clip.py — 20/20 passing
• Verify notebook renders correctly
• Verify docs build with make html