PR preview builds

can't use vercel right now because we have separate static sites for each year subpath, not a single project.

we could create a new GH user as the PR-building bot and using GH actions, build the static site for each PR, create a new repo under the bot user (named `pr-{{ year }}-{{ pr number }}`), enable GH pages on the new repo, and then comment on the PR with a link to the created repo (which has built files) and a link to the actual site