Hugo offers a couple of options to include diagrams right in the source code, see https://gohugo.io/content-management/diagrams/
Here’s a mermaid example: (note the hasMermaid = true parameter in the front matter!)
classDiagram
Animal <|-- Duck
Animal <|-- Fish
Animal <|-- Zebra
Animal : +int age
Animal : +String gender
Animal: +isMammal()
Animal: +mate()
class Duck{
+String beakColor
+swim()
+quack()
}
class Fish{
-int sizeInFeet
-canEat()
}
class Zebra{
+bool is_wild
+run()
}