https://github.com/GitbookIO/plugin-puml
Then in your content:
This is a diagram:
{% plantuml %}
Bob-
>
Alice : hello
{% endplantuml %}
The plugin will replace the{% plantuml %}by SVG images (and PNG images for ebook output).
This is a diagram:
{% plantuml %}
Bob-
>
Alice : hello
{% endplantuml %}