docbook-xsl (used for man page generation) is orphaned

Hi,

while hunting down man page errors (warning: name 'an-trap' not defined) I noticed that they are still generated with docbook-xsl. Would it make sense to generate them through asciidoctor (or some other tool) since docbook-xsl has been abandoned for nearly a decade now? I'd be willing to create a PR to update the build system.

Greets, Lee