message: extend about/download page with new static root
diff: subfolder.
  </p>
  <p>
  - Docs.rs is running rustdoc with <code>--static-root-path "/"</code>,
  + Docs.rs is running rustdoc with <code>--static-root-path "/-/rustdoc.static/"</code>,
  which leads to all references to static assets breaking if they are not
  - available under that path.
  + available under that path. Older builds used <code>--static-root-path "/"</code>, which
  + means you will have to handle both.
  </p>
  <p>
  Since we're also adding <code>--emit=invocation-specific</code> to our build
  