Provide example with versioned assets

[mdedetrich]

An example is provided on how to use standard assets, but it would be great to show how to serve versioned assets using something like https://github.com/sbt/sbt-digest

[mdedetrich]

I have attempted to try and implement this myself as a plugin however I have hit a roadblock which you can see here https://github.com/mdedetrich/akka-http-with-scalajs-digest-issue

[mdedetrich]

I have made a project here https://github.com/mdedetrich/sbt-digest-reverse-router which should greatly simplify this.

I will wait until sbt/sbt-digest#29 is merged and sbt-digest is released as a new version, and then I will make a new PR in this folder which demonstrates versioned assets using the library