Only using gallery feature but thumbnails do not show up

[twiecki]

For PyMC we're using myst_nb, but we'd like to use the gallery feature from nbsphinx. There is a hack that allows to use the two in tandem and only use gallery feature from nbsphinx. That works and creates a gallery, however, it doesn't extract thumbnails specified via metadata tags. It seems like the NB is never actually parsed.

However, it does work if I specify the thumbnails directly in conf.py via nbsphinx_thumbnails.

I'm posting here to get some help in case it's obvious to anyone that it's not working because I forgot to X.

Here's the current PR: pymc-devs/pymc-examples#328
Here's the PR I got the hack from: PlasmaPy/PlasmaPy#1474 (comment)
Related issue: executablebooks/MyST-NB#396