Route logs through the Asciidoctor logger

[ggrossetie]

• Reduce log severity to info (previously was warn)

Here's an example when running Antora 3:

$ npx antora --log-level=info site.yml
[15:05:03.622] INFO (asciidoctor): Skipping preprocessing of PlantUML standard library include '<C4/C4_Container>'
    file: /path/to/dir/docs/modules/ROOT/pages/topic/index.adoc
    source: /path/to/dir (refname: main <worktree>, start path: docs)

Ref #147