docs: clarify rewrite notebook guidance

[binaryaaron]

Follow-up to #218.

Summary

• Restore file-level SPDX headers in all five Python notebook sources while retaining the hidden notebook notices.
• Describe repair and human-review decisions as separate threshold checks.
• Calibrate rewrite privacy language and clarify the leakage-metric comparison.
• Synchronize the affected notebook input cells without changing execution outputs.

Validation

• tools/codestyle/format.sh --check
• tools/codestyle/ruff_check.sh
• tools/codestyle/copyright_fixer.py --check .
• mkdocs build --strict
• Notebook schema validation for all five tutorials
• Source/notebook input-cell synchronization check