(feat): generate preview URLs for each pull request

[joshnatis]

This will help contributing students preview their contributions without having to run anything locally (and similarly, will help us review contributions without needing to sync them to our computers).

There is a "pull request preview" feature currently in private alpha over at https://github.com/actions/deploy-pages (which we use to power our build and deploy process). I left a comment on their pull request with the feature expressing our interest/excitement.