Move AsciiDoc attribute "manmanual" from the Makefile to the adoc files

It is simpler to just specify the static attribute in the documents themselves.

Makefile: support and prefer Asciidoctor

Asciidoctor is faster in generating man pages and also uses a sensible date format (e.g. "2024-02-16" instead of "03/17/2024" like a2x does).

a2x does not need the -d option since the doctype is set in each .adoc file.