Make use of Mermaid Diagrams

[LukeTowers]

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.

[mjauvin]

Where do we use diagrams in our Docs ?

[LukeTowers]

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

[bennothommo]

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
```