Manually add Pythia content to gallery

[kmpaul]

While #126 is all about automated addition of Pythia content to the gallery, we should start be looking at manually adding our content to the gallery first. This may inform how we want to automate this process in the future, but we should get our content on the gallery as soon as possible.

[brian-rose]

Probably best to merge #160 (tags for affiliation) first, so we can add a "Pythia" tag to each content piece we add.