Add thumbnail images to the external resources page

[seisman]

What do people think about adding thumbnail images to this external resources page? I'm thinking of using 'cards' like in the gallery/team/overview page, and having a thumbnail of the linked repository or youtube page so people know what is in those links before they click. Can be done in a separate PR.

Originally posted by @weiji14 in #1935 (review)

[seisman]

Perhaps we can use the sphinx-design extension's card directive (https://sphinx-design.readthedocs.io/en/latest/cards.html).