Docs finetuning callback example

[SkafteNicki]

What does this PR do?

Fixes #19711
Adds documentation on how to use the BackboneFinetuning callback as it seems to confuse users how to format their model to work with the callback.

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📚 Documentation preview 📚: https://pytorch-lightning--21216.org.readthedocs.build/en/21216/