[Documentation]: Add diagrams with mermaid

[Mark90]

Context

As part of the initiative to improve the WFO documentation, we want to have diagrams to explain things in a visual way.

Proposed solution

The repository already has dependency pymdown-extensions which provides a way to include Mermaid diagrams in the documentation.

For example, this markdown snippet defines a flowchart:

```mermaid
graph LR
  A --> B
  B --> C
  C --> A
```

When not escaping the backticks it renders in Github automatically:

graph LR
  A --> B
  B --> C
  C --> A

But most importantly, it automatically renders in a mkdocs build or in the interactive mkdocs serve:

A few random examples of diagrams we could add (what we really want should be refined):

• Entity diagrams for Database tables (Reference docs)
• Flow/activity diagrams for process handling (Architecture docs)
• Systems diagram for how WFO integrates (Architecture docs)

Tasks

• [ ] Add pymdown-extensions as explicit docs dependency
• [ ] Explore mermaid diagram types
• [ ] (to be refined) Add diagrams in Architecture, Guides and Reference docs where applicable

[Mark90]

Might be nice to have a diagram for the websocket manager architecture: https://workfloworchestrator.org/orchestrator-core/reference-docs/websockets/?h=redis#implementation