Add PR preview support

[wborn]

It would be nice to be able to preview documentation changes via previews.
This helps with reviewing PRs that make significant changes such as #59.

Native support in GitHub pages is still being worked on, see https://github.com/orgs/community/discussions/7730.

Similar functionality is already available when using Netlify or Firebase, see:

• https://github.com/FirebaseExtended/action-hosting-deploy?tab=readme-ov-file#deploy-to-a-new-preview-channel-for-every-pr
• https://docs.netlify.com/site-deploys/deploy-previews/