docs: add header comment for replace_logger script

[RohitMandal-cs]

##Summary

• Added a header comment explaining the purpose of backend/replace_logger.cjs.
• Documented that it is a one-time migration script and not part of the application runtime.

Fixes #1507

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• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation accordingly.
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