Move catalog thumbnails to their respective repositories

[vincerubinetti]

Just made this issue about thumbnails being stored in the manuscript repo itself:
manubot/rootstock#267

As such, we should eventually move the thumbnails that are right now stored in this repo to their respective repos. This will also be a good excuse to update the rootstock theme for all those manuscripts, because I see that some of the papers are using old CSS with glitches.

For the papers that we don't directly control, I can even submit PRs for.

Once we do this, we can change the catalog script to fetch the thumbnails from the repos instead (hopefully that is easy).