docs: fix example notebooks

[oboulant]

This PR fixes the docs job in the CI : https://github.com/deepcharles/ruptures/actions/runs/3826472378/jobs/6510297491

The underlying reason is that the API of sklearn's CountVectorizer has changed (see here for change).

get_feature_names is now deprecated and becomes get_feature_names_out.

Edit

Actually docs/examples/music-segmentation.ipynb and docs/examples/kernel-cpd-performance-comparison.ipynb were not passing also. Had to run the nbqa-black pre-commit locally.

Moreover, I had to re-order imports in docs/examples/music-segmentation.ipynb because (I guess), something was shadowing matplotlib.something.

[deepcharles]

Thanks!