Fix missing thumbnails for notebook gallery

[fziegner]

This PR addresses #426. The thumbnails are now saved in the _static directory instead of being selected from the notebooks. This also makes it possible to choose specific thumbnails for the gallery. Some of the thumbnails are still suboptimal so I'll be looking for a way to improve them.

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[github-actions[bot]]

Linting results by Pylint:

Your code has been rated at 8.69/10 (previous run: 8.69/10, +0.00) _{The linting score is an indicator that reflects how well your code version follows Pylint’s coding standards and quality metrics with respect to the RC_v1.5.x branch. A decrease usually indicates your new code does not fully meet style guidelines or has potential errors.}

[fsenf]

https://tobac--428.org.readthedocs.build/en/428/

[fsenf]

I included a colorful set of thumbnail images. Hope that looks nice now ...

[fsenf]

@w-k-jones : this documentation piece is ready. Would you like to act as 2nd reviewer?

PS: Unfortunately, the website under the link above did not update to show the newest image versions....