[docs] fix links to kak decomposition demo

[Qottmann]

following PennyLaneAI/qml#1287 and PennyLaneAI/qml#1294
cc @dwierichs @justinpickering

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

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[Qottmann]

For as long as the objects.inv file is not updated I would proceed with this hot fix that uses static links - for the moment as long as the :doc: links to the new demos continue to not work

[PietropaoloFrisoni]

Once you pull from the docs branch, there should be an automatic sphinx check. If that is fine this is good to go for me!