Use asciidoctor instead of asciidoc-py for manpage generation (#4534)

- configure.ac: replace asciidoc+xsltproc checks with asciidoctor check,
  remove HAVE_XML_CATALOG_FILES conditional (not needed with asciidoctor)
- Makefile.am: use asciidoctor -b manpage for man pages, asciidoctor -b
  html5 for HTML, asciidoctor-pdf for PDF; remove man_xslt variable and
  docbook intermediate steps
- doc/generate_manpages.sh: replace asciidoc+xsltproc with asciidoctor;
  use separate variable for asciidoctor-pdf; make PDF optional if
  asciidoctor-pdf is not installed
- ci: build and upload docs as artifact in autotools workflow

Co-authored-by: amitdo <13571208+amitdo@users.noreply.github.com>
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Signed-off-by: Stefan Weil <sw@weilnetz.de>