Docs: highlight key optimization code lines with :emphasize-lines:

[tsubasakong]

What does this PR do?

Fixes #12119.

This PR adds Sphinx :emphasize-lines: directives to key Python examples in the optimization docs so readers can quickly spot the important lines:

• Automatic optimization loop internals (closure / step points)
• Accessing the wrapped optimizer vs. raw optimizer
• Custom LR scheduler integration
• Gradient clipping customization
• estimated_stepping_batches usage with OneCycleLR

These are docs-only changes (no behavior changes).

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📚 Documentation preview 📚: https://pytorch-lightning--21568.org.readthedocs.build/en/21568/

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Checklist

• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you verify new and existing tests pass locally with your changes?