Fix: documentation build by upgrading Sphinx to version 5 and replacing deprecated sphinx-panels with sphinx design

[Rishabhsaini21]

Description

This PR fixes the documentation build failures related to Sphinx and deprecated extensions.

Affected Dependencies

Changed docs/requirements.txt and docs/conf.py

How has this been tested?

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Checklist

• [ ] I have followed the Contribution Guidelines and Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests