doc: use asciidoctor instead of a2x

AsciiDoc development is continued under asciidoctor. See
https://github.com/asciidoc/asciidoc.

We do however fallback to a2x if asciidoctor is not present. This is to
ease migration, but at some point, it's likely that support for a2x will
be dropped.

Originally reported downstream:
https://github.com/Homebrew/linuxbrew-core/issues/19885

Closes #1544

ci/ubuntu-install-packages

@@ -2,5 +2,5 @@
 
 sudo apt-get update
 sudo apt-get install -y --no-install-recommends \
-  libxslt1-dev asciidoc docbook-xsl xsltproc libxml2-utils \
+  asciidoctor \
   zsh xz-utils liblz4-tool musl-tools