📚 Update RKDE lighting model file header (hence docs) with paper URL

[tobybreckon]

📝 Description

Add URL to paper here also so that it also appears in readthedocs reference page at: https://anomalib.readthedocs.io/en/latest/markdown/guides/reference/models/image/rkde.html

✨ Changes

Select what type of change your PR is:

• 📚 Documentation update

✅ Checklist

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• [SKIPPED - minor ] 📋 I have summarized my changes in the [CHANGELOG] -
• [X ] 📚 I have made the necessary updates to the documentation (if applicable).
• [ N/A ] 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).

[samet-akcay]

Nice :)

[ashwinvaidya17]

Thanks!