DOC: Add reference to PlantUML GitLab repository

docs/infrastructure/plantuml.rst

@@ -3,6 +3,10 @@ PlantUML
 
 We're running a `PlantUML Server <https://plantuml.confirm.ch/>`_ to create our own `PlantUML <https://plantuml.com/>`_ diagrams.
 
+.. hint:: 
+
+    The PlantUML server is deployed in the `PlantUML Server GitLab project <https://git.confirm.ch/confirm/plantuml>`_ and is only available from within private networks.
+
 Use PlantUML
 ------------