Default Jupyter notebooks thumbnails

[arnaucasau]

Summary

This PR adds a default thumbnail for the Jupyter notebooks. In the following screenshots we can see the change:

Without a default thumbnail:

With a default thumbnail:

[grace-harper]

Hey @arnaucasau Is it possible to have the default thumbnail show only when there are not pre-existing thumbnails for the tutorial?

[arnaucasau]

Hi @grace-harper, yes! With the changes in this PR, the default thumbnail will be used only if you don't set any other thumbnail. There are multiple ways to set the thumbnails, you can use an image displayed in the tutorials by adding the tag nbsphinx-thumbnail to the cell with the image, or you can modify nbsphinx_thumbnails in theconf.py file by adding an entry below the one I added with the name of the notebook and the image you want to set as thumbnail.

[grace-harper]

@arnaucasau On my end, it looks like Qiskit QEC Software Framework thumbnails gets overwritten by the default.

[arnaucasau]

Hmm, you're right. I added the tag in the images used as a thumbnail before. I think all of the notebooks have recovered their thumbnails now, and the other ones have the default. Thank you @grace-harper :)

[grace-harper]

Awesome! Thanks!