Allow PR Previews

[ES-Alexander]

Following on from #1, our current attempted preview mechanism doesn't work.

For context:

PRs (logically, in hindsight) normally cannot access secrets, so we need to put some more thought into how we want to generate previews in a safe manner.

Generating and uploading an Artifact seems like a viable first step, but because that doesn't host the docs it's a bit cumbersome. Following the github security resources that's the first step for limiting untrusted access anyway, and then we likely want to upload to a separate AWS bucket, potentially with extra restrictions on what kind of actions the files can perform (e.g. we may wish to restrict javascript to prevent malicious code execution for testers).