Docs: use "Visual diff" for marketing "Doc Diff" (#11860)
Related https://github.com/readthedocs/readthedocs.org/pull/11838
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:books: Documentation previews :books:
- User's documentation (`docs`):
https://docs--11860.org.readthedocs.build/en/11860/
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- Developer's documentation (`dev`):
https://dev--11860.org.readthedocs.build/en/11860/
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