Lint notebook code

[frankharkins]

Summary

• Added tests to check notebook code cells with Black & Pylint
• Cleaned up the notebooks to pass the tests
• Added info to the contributing guide

See #1393 for more information

Notes

I've hidden some Pylint comments by adding them to cells with metadata:

"nbsphinx": "hidden"

inline comments will show on the live site, there doesn't seem to be an easy way to remove them at build time.

[review-notebook-app[bot]]

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[HuangJunye]

We decided to put this on hold until we figure out the future of the qiskit-tutorials repo.