[ENH] Fix thumbnails gallery

[acocac]

Summary

The PR aims to have a gallery with fixed size thumbnails imported from notebook repositories.

Fixes #231

List of changes proposed in this PR (pull-request)

• Update URLs of notebook thumbnails
• Update submodules to the latest commit

What should a reviewer concentrate their feedback on?

• [ ] Everything looks ok?

Acknowledging contributors

• [x] All contributors to this pull request are already named in the table of contributors in the README file.
• [ ] The following people should be added to the table of contributors in the README file:

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