docs: update favicon to match Lightning AI app consistency

[EmilieLny]

What does this PR do?

This PR updates the favicon of the repository to match the one used in the Lightning AI app, ensuring consistency across our branding. The favicon includes several sizes (16x16px, 48x48px, 224x224px) for optimal display.

BEFORE:

AFTER:

Lightning ai's home page:

Fixes #<issue_number>

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📚 Documentation preview 📚: https://pytorch-lightning--20297.org.readthedocs.build/en/20297/