feat: Validate links not specified with `link`

[jboxman]

This would be a wholesale enhancement, and I don't know how common it might be. It isn't just listing blocks; This could happen for links in code font as well, unless Asciidoctor also transforms those.

[subs="attributes"]
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oc create -f https://raw.githubusercontent.com/jboss-fuse/application-templates/application-templates-2.1.0.fuse-sb2-780019-redhat-00005/fuse-prometheus-operator.yml
----

Does it make sense to parse the output HTML for links instead and not pull them from Asciidoctor node objects? The problem with this is that a broken link cannot then be traced by to a specific file and line number.