After testing a previous revision, I discovered internal xrefs generated with crossref: do not work at all on PDF [0].
This revision updates all of the internal links in this chapter to use the syntax instructed in Asciidoctor official doc on xrefs [1].
I am currently going through the rest of the handbook for these while digesting the considerations from x11 refactor, this seems like a great task for me.
[0] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266107
[1] https://docs.asciidoctor.org/asciidoc/latest/macros/xref/