Fix link to download notebook badge in documentation

[bennibolm]

The old link to the download badge doesn't work anymore. This PR replaces the link with a new self-generated one.
To prevent this from happening in the future for the link to the nbviewer badge, I updated it as well.

New badges are created directly with https://shields.io/:
https://img.shields.io/badge/render-nbviewer-f37726
https://img.shields.io/badge/raw-notebook-4cc61e

[github-actions]

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[sloede]

Thanks for providing a fix! Instead of storing the badges here, you could also just use the shields.io links directly. If those cease to work, we are in trouble anyways.

[efaulhaber]

Note that if shields.io links are used anyway, the advantage of using them everywhere is that one can easily modify the link to change the shown text or color.

[bennibolm]

Note that if shields.io links are used anyway, the advantage of using them everywhere is that one can easily modify the link to change the shown text or color.

🚜

[bennibolm]

Thanks for the suggestion. I changed it in e339958.

[sloede]

LGTM, thanks!