The diagrams in the application documentation are PlantUML sources kept next to their rendered SVGs in docs/applications/*/diagrams/, sharing the style.puml skin in the same folder. Edit the .puml, re-render, and commit both files:
scripts/render_diagrams.py
Rendering uses a PlantUML server, so you do not need a local Java installation, and !include lines are resolved before the source is sent. Pass paths to render only some diagrams, and --server to use a local server instead of the public one:
docker run -d -p 8080:8080 plantuml/plantuml-server:jetty
scripts/render_diagrams.py --server http://localhost:8080
A source that does not parse fails with the error and line number the server reports. The notation the diagrams use is documented in Diagram notation.