* migrate from the old, unmaintained "asciidoc-py" tool to the new "asciidoctor" generator
* migrate from asciidoc-py syntax to the modern Asciidoc syntax (especially page titles and section titles)
* remove the need of "xmlto" utility to create the manpage output; use asciidoctor for that
* add HTML output support to the doc/Makefile by using asciidoctor
* change API documentation files extension from .txt to .adoc to make it more explicit that they are Asciidoc-encoded (as a bonus several IDE plugins will autodetect the .adoc format as Asciidoc)
* remove asciidoc.conf: asciidoctor does not support that; this also required replacing the macro linkzmq into all documentation pages
* add a new Github action CI do deploy to Github Pages the static HTMLs produced by Asciidoctors
* removed references to the "xmlto" and "a2x" tools from the build and packaging systems: Asciidoctor can convert the documentation directly to e.g. pdf (via extended converters) and anyway there was no code/target for using "xmlto" and "a2x" tools anyway
