Try with mermaid instead of plantuml

docs/architecture/diagrams/model_timeline.md

@@ -19,7 +19,7 @@ A more complete diagram of the data models, their fields, and their
 relationships are in [models_diagram.md](./models_diagram.md), created with
 the `django-model2puml` plugin.
 
-```plantuml
+```mermaid
 @startuml
 
 allowmixing

docs/architecture/diagrams/models_diagram.md

@@ -8,7 +8,7 @@ using the command
 $ docker compose app ./manage.py generate_puml --include registrar
 ```
 
-```plantuml
+```mermaid
 @startuml 
 class "registrar.Contact <Registrar>" as registrar.Contact #d6f4e9 {
     contact