This PR adds a `deploy.yml` to match [what's in our template repository](https://github.com/just-the-docs/just-the-docs-tests). I think it would be a great idea for us to match parity, especially:

- so that it's easier for us to help users & debug
- so that if we move to Jekyll 4 (likely soon), the transition is seamless
- so that we can explore adding custom items to the build step

Before merging this, we'll also need to change the source of the Pages deploy to actions (from the current branch deploy).