[doc] document how to run generate_all_images.py

Author: jjimenezshaw

docs/docbuild/README.md

@@ -6,3 +6,9 @@ From the base PROJ repository directory:
 cd docs
 docker run -i -t -v $PWD/..:/io -w /io/docs/ ghcr.io/osgeo/proj-docs make html
 ```
+
+When a projection image is added or deleted run
+
+```bash
+python3 docs/source/operations/projections/generate_all_images.py
+```