[ci skip] Add example thumbnails

[michalk8]

This is the example @Marius1311 requested, see also here: https://nbsphinx.readthedocs.io/en/rtd-theme/subdir/gallery.html I'd prefer to use the path in _images, since it is more robust than modifying the code cell's metadata and it doesn't create extra files if we were to manually place them in _static/img.

If you agree, please remove the last commit introducing the temporary Untitled.ipynb that was just for showcase and update nbsphinx_thumbnails accordingly.

closes #903

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[Marius1311]

Thanks for adding this, will get back to this over the coming days.

[michalk8]

closing this because we will switch to myst-nb

[Marius1311]

ok, but can we still have nice Thumbnails?