Make use of Mermaid Diagrams

We should update our docs to make use of Mermaid diagrams: https://mermaid.js.org/config/usage.html. Would need to add support in the Nabu theme as well as update the doc content itself to include the diagram definitions where applicable.

Where do we use diagrams in our Docs ?

That's the point, we should use it more @mjauvin 😂 Especially in the database / relationships section.

OMG I need this.

Should be easy to implement, I think we would just use a simple code block to surround the Markdown:

```mermaid
    ... graph code here
```