Doc: Add DTM tutorial

[geographika]

A first draft of a tutorial working with a DTM and the GDAL raster commands. Tested on Linux and Windows (PowerShell), using the "traditional" tools and the new GDAL CLI. A few additional glossary items have also been added. Images included in the docs have been shrunk to keep the doc size down. Both the traditional tools and GDAL CLI worked flawlessly.

Preview online at https://gdal--12792.org.readthedocs.build/en/12792/tutorials/raster_dtm_tut.html

Please let me know if there are any errors / misconceptions / better ways to do things.

Notes/questions:

• The geometry type needs to be set with -nlt POLYGON using ogr2ogr. This does not seem to be necessary using gdal vector pipeline
• I think --allow-bbox-outside-source is required as the projections for the vector and raster datasets differ (rather than the polygon is outside the raster extent).
• The final image would look better with a white background. I think this would require creating a new image?

[dbaston]

The geometry type needs to be set with -nlt POLYGON using ogr2ogr. This does not seem to be necessary using gdal vector pipeline

I would add set-geom-type --geometry-type MultiPolygon to the pipeline for consistency. If you leave the geometry type as-is (MultiSurface) then gdalwarp will complain about it.

The final image would look better with a white background. I think this would require creating a new image?

Or a transparent background? I think it's OK to manipulate the images for display.

Do we have any concerns about the image sizes?

[geographika]

Thanks @dbaston for the thorough review - I've updated the PR with your suggestions.

I would add set-geom-type --geometry-type MultiPolygon to the pipeline for consistency. If you leave the geometry type as-is (MultiSurface) then gdalwarp will complain about it.

Done.

The final image would look better with a white background. I think this would require creating a new image?

Or a transparent background? I think it's OK to manipulate the images for display.

Do we have any concerns about the image sizes?

I have updated to use a transparent background, and reduced vertical exaggeration - and much happier with the output!
All images for the docs have been reduced in size and compressed using pillow. In total, all images for the tutorial are 374 KB.