Link to preview deployments

[KaiserKarel]

If a PR alters the docs, we should do a preview deployment to vercel and show the preview URL in a comment. This should be possible by checking if nixbuild.net built the docs, or if it already had it in cache. If built, the docs should be fetched and uploaded.

This can be achieved with something like

run: |
  nix copy --from nixbuild.net $(type -p ./#docs)
  vercel build
  vercel deploy > url.txt
working-directory: docs  
continue-on-error: true    
run: |
  if test -f url.txt; then
    create comment
  fi

With this, we'll get links to previews only if the docs changed. A downside is that the nix store can remove cached docs if they haven't been built in a while. Alternatively we can also only run the preview URL if the docs directory is changed; but that however means that if docs depend on anything outside of the docs directory, we might build broken docs without knowing it.

[KaiserKarel]

We're now showing preview URLs, but on every single pull_request. Next a check should be added to not preview if the input hash of main == hash of PR.