Ruby 3.2 has stabilized (and the bug with Liquid has been patched). I think this is a good time for us to bump all of our versions, especially with the discussion in #1307.
Separately, I've:
- changed the core test matrix to be across `3.0`, `3.1`, and `3.2`. Ruby 2 has been EOL for ~ 4 months, while 3.0 is around for about 7 more months.
- standardized the formatting of our CI ruby versions to be strings; this is because the number `3.0` gets YAML-casted to `3`
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).
