feat: add visualization of the abstract MFA structure

[tobiasdiez]

Purpose of this PR

Add two ProcessGraphPlotter classes (using either plotly or graphviz) for visualizing MFA system structures.

Also updated some of the documentation/examples to demonstrate the new plotting capabilities.

A few examples (processes are in blue, stocks in green, flows as edges):

• Example 1 (plotly)

- Example 2 (plotly) - Example 2 (graphviz) - remind-mfa (steel with graphviz)

Disclaimer: the layout algorithm for plotly was mostly created using Claude

Checklist:

• I have used the Conventional Commits format for my PR title (and commit messages)
• I have updated the in-code documentation of all changed classes and functions
• I have added in-code documentation and type hints to all new classes and functions
• I have adapted the howtos
• I have adapted the examples

[JakobBD]

That's a great feature!

Is there a way in either of the two to pass prescribed x and y positions of the nodes?
Best would be minimal grid integers, i.e. (0,1) instead of sth like (354,762).
Should be optional, of course.

Also, in graphviz, is there a way to have uniform node sizes?

Regardless, the feature is already great and good to merge!