Add `nbsphinx` thumbnails to talktorial gallery

[dominiquesydow]

Procedure

Procedure for adding nbsphix thumbnails to the gallery:

• Open talktorial with JupyterLab
• Open "Property inspector" menu on the right site of the browser
  
• Go to the cell with the output that shall be used as thumbnail and tag the cell as shown in the image above; you may need to type "nbshpinx-thumbnail" to the "Add tag" window by hand.
• Note: In our approach, we do not use image files as thumbnails but images that are output in a Jupyter notebook
• Note: If you inspect the plain text of a tagged notebook, the metadata tag you will find is:

   "metadata": {
    "tags": [
     "nbsphinx-thumbnail"
    ]
   },
  

Resources

• Sphinx Gallery: https://sphinx-gallery.github.io/stable/index.html
• NBSphinx: https://nbsphinx.readthedocs.io/en/0.8.7/
• Both together: https://nbsphinx.readthedocs.io/en/0.8.7/subdir/gallery.html