docs: fix broken links surfaced by the weekly linkcheck cron

[brendancol]

The weekly docs linkcheck cron (run 27531921329, 2026-06-15) is red with 7 problems. Most are external links that have rotted, a couple are anchor fragments linkcheck can't resolve, and the new landing index page reported some broken internal links. Listing them so the weekly cron can go green again.

External links that 404 or are blocked

• user_guide/focal (line 204): http://www.jennessent.com/downloads/tpi-poster-tnc_18x22.pdf — 404
• user_guide/surface (line 303): http://wenatcheeoutdoors.org/2016/04/07/avalanche-abcs-for-snowshoers/ — 404
• user_guide/proximity (line 682): https://pro.arcgis.com/en/pro-app/tool-reference/spatial-analyst/an-overview-of-the-distance-tools.htm — 404 (Esri moved these under doc.esri.com/en/arcgis-pro/...)
• user_guide/surface (line 895): https://pro.arcgis.com/en/pro-app/tool-reference/spatial-analyst/an-overview-of-the-surface-tools.htm — 404 (same Esri move)
• xrspatial.proximity.allocation docstring (line 81): https://github.com/OSGeo/gdal/blob/master/gdal/alg/gdalproximity.cpp — 404 (GDAL repo restructured; the file lives at a different path now)
• xrspatial.multispectral.savi docstring (line 25): https://www.sciencedirect.com/science/article/abs/pii/003442578890106X — 403 (ScienceDirect blocks the checker; likely an ignore-list entry rather than a URL change)

Anchor fragments linkcheck can't resolve

• xrspatial.proximity.euclidean_distance docstring (line 18): the Wikipedia Euclidean_distance link uses a #:~:text= text fragment
• xrspatial.proximity.great_circle_distance docstring (line 23): same, on Great-circle_distance
• getting_started/why_xarray_spatial (line 27): https://github.com/xarray-contrib/xarray-spatial#readme — anchor readme not found

Broken internal links from the new landing index (#3316)

These are internal targets, not external rot, so probably a separate fix:

• user_guide/classification.html, user_guide/multispectral.html, user_guide/proximity.html, user_guide/surface.html
• user_guide/surface.html#Hillshade, user_guide/surface.html#Slope
• the examples/pharmacy-deserts.ipynb anchor "Create-a-Distance-to-Nearest-Pharmacy-Layer-&-Classify-into-5-Groups"

Notes

Linkcheck only runs on the Monday cron and on manual dispatch (not per-PR), and it's allowed to fail loudly, so it stays red until these are fixed. For the text-fragment and bot-blocked cases, a linkcheck_ignore / linkcheck_anchors_ignore entry in conf.py is probably the right fix rather than editing the URL.