Allow prerendering diagram for asciidoctor templates

[jtama]

The asciidoc-java doesn't support diagram blocks, and don't plan to.

However they offer extendable parser that allows pre-rendering (see yupiik/tools-maven-plugin#23).

I have prepared a PR that follows that path.

[jtama]

This is has been achieved in quarkiverse/quarkus-roq#536.

It leverages kroki.io.

Feel free to close the issue.