add/change code to run on simulator

[romeokienzler]

fixes #4281

I've realized it is hard to work with the notebooks since it is not explained how and where to obtain the token/crn.

I've added this explaination and also added a fallback to the simulator in case the token is not set

Also the free backend doesn't seem to support the 'with Session:' directive, therefore I've changed that code block as well.

The notebook now runs fine up to 'Part II. Scale it up!' since 'backend.coupling_map' is called and neither the AerSimulator nor the free tier backend seem to support it

[qiskit-bot]

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One or more of the following people are relevant to this code:

• @a-matsuo
• @annaliese-estes
• @miamico

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[romeokienzler]

closing as rejected by @miamico