docs(factory-method): enhance README with class diagram + diagram heading (fixes #3366)

[oldregime]

This PR improves the Factory Method pattern documentation:
    Adds an explicit Class diagram section (aligns with localized docs and other patterns)
    Normalizes the Sequence diagram heading to H2
Fixes #3366
Docs-only change; no code modified.

[github-actions]

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• 4d1505c: docs(factory-method): add class diagram section and normalize sequence diagram heading (demo file Report #3366)

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