Fix missing resources in asciidoc output

[SimonMarquis]

The web page does not export images, and we get a broken link.

I choose the solution to not move resources in a dedicated directory since it is already used on the main README. But this could be changed if needed.

[SimonMarquis]

FYI, the default expected directory containing images would be asciidocs/images as documented in https://asciidoctor.github.io/asciidoctor-gradle-plugin/development-3.x/user-guide/#_processing_auxiliary_files

If resources is never set, the default behaviour is as if the following was called

resources {
  from(sourceDir) {
    include 'images/**'
  }
}