docs: add prepare_for_model() v4→v5 migration example

[MushiSenpai]

What does this PR do?

Fixes # (issue)

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[github-actions]

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CI Security Gate — automatic approval blocked

This PR was not automatically approved for CI because the security gate failed.

Possible reasons:

• The PR touches 50 or more files — only PRs with fewer than 50 changed files are automatically approved
• A changed file is outside the allowed directories (src/, tests/, docs/, utils/), has a disallowed extension (only .py, .txt, .md permitted outside tests/ and docs/), or is not .md/.yml inside docs/
• A new high-severity security issue was detected in the changed Python files (Bandit check)

See the workflow run for the exact violations.

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