Automate updating requirements for notebook testing

[Eric-Arellano]

Basically: set up dependabot for https://github.com/Qiskit/documentation/blob/main/scripts/nb-tester/requirements.txt. We want to always make sure we are testing the latest stable release of the docs.

Per #133, we should also test other versions of Qiskit when relevant, e.g. Qiskit 1.0 if the latest has been set to 2.0. But that can be a separate configuration.

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If dependabot is not viable, then we could consider unpinning the versions in requirements.txt, but this is not desirable because it makes our CI undeterministic. It's far better to only change the requirement via a dedicated PR.