[documentation]: Fix links in all output formats

Use the crossref: macro for intra-file and intra-book links.
This will produce the correct link in Single HTML, Splitted HTML and PDF.

Rework the macro to include the PDF special render.

While here, briefly document macros.

PR: 266107
Reviewed by: bcr@, dbaio@, concussious.bugzilla_runbox.com
Differential Revision: https://reviews.freebsd.org/D46480