set up live previews for prs

[jyn514]

we have a static site built from markdown sources. as a reviewer it would be nice to easily view the rendered html for a PR, not just the raw markdown.

not sure who to ask about this - maybe t-infra? maybe we could set up rust-lang.github.io/rustc-dev-guide or something about that?

[jyn514]

https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/live.20previews.20for.20rustc-dev-guide.3F/with/520974503