Implement reference previews

[marcoieni]

Set up rendered previous or PRs for the reference repository.

Zulip proposal

Zulip discussion here.

1. Set up a static website with an S3 bucket to host the content on.
2. Add the necessary secrets to the reference repo so that a GitHub action can upload the files to the bucket (use oidc instead)?
3. Add a way to trigger the preview (i.e. store the files to the S3 bucket). Example: this can be done from a workflow_dispatch or from a GH comment in the PR. Example here.

• The intent is for these previews to only be generated manually via a bot by a team member. They would type something like @rustbot preview in the GH comment).
• The action that generates the preview will only be on the master branch, so a PR can't change what s3 command is run.

My proposal

Use github pages. Demo here.

[marcoieni]

done in rust-lang/reference#1647