Add docs page for custom voxel statistic rasters

[iosefa]

Summary

Adds user-facing documentation for generating custom raster products from
arbitrary point dimensions using calculate_voxel_stat, including a concrete
example for mean Intensity at 1 m resolution.

Changes

• Added new usage page: docs/usage/voxel-stat-rasters.md
  • Explains the calculate_voxel_stat + create_geotiff workflow
  • Includes example: mean Intensity at 1 m XY resolution
  • Lists supported statistics (mean, sum, count, min, max, median, std)
  • Shows how to restrict aggregation with z_index_range
• Updated docs navigation in mkdocs.yml to include:
  • Usage -> Custom Voxel Statistic Rasters

Why

calculate_voxel_stat existed in the API but did not have a clear usage guide
in the docs/README for arbitrary dimension aggregation to raster outputs.

Validation

• Confirmed docs render locally with mkdocs serve
• New page is accessible under Usage in the nav

Notes

• Docs-only change; no package/release changes required.

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