Plugin dev: consider removing mermaid class diagram in Part 2

[pinin4fjords]

From PR #761 review by @adamrtalbot:

I don't think this graph adds anything useful to me?

The mermaid diagram in Part 2 (02_create_project.md, section 4) shows the relationship between GreetingPlugin, GreetingExtension, GreetingFactory, and GreetingObserver. Adam's concern is that it may not add value at this point in the lesson since learners haven't yet worked with these classes.

Options:

• Remove it
• Move it to a later part (e.g., Part 5 after observers are introduced)
• Keep it as a reference but collapse it in an admonition

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