Asciidoc default css

[ia3andy]

Figure out a way to have a default css for asciidoc that can easilly be opted-in or out with config

[ia3andy]

There is a asciidoc default css:
https://cdn.jsdelivr.net/gh/asciidoctor/asciidoctor@2.0/data/stylesheets/asciidoctor-default.css

and also an antora ui one which I personally prefer (the one used in iamroq).

We should make sure it is isolated from the user css with a asciidoc parent class or something.