Thumbnails for the new gallery entries are missing

[fsenf]

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With #408 we got cool new gallery entries, see https://tobac--408.org.readthedocs.build/en/408/examples.html .

Some creative mind could design some cool thumbnails for these entries (the notebooks itself create no figures ... something external needs to be added).

Workflow

1. add thumbnails here: https://github.com/tobac-project/tobac/tree/main/doc/_static/thumbnails
2. provide mapping between thumbnails and notebook in conf.py, here: https://github.com/tobac-project/tobac/blob/698d53bf019de6542056272c01d6a472caaf2c30/doc/conf.py#L84