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/macos-install-packages

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-brew install asciidoc docbook-xsl
+brew install asciidoctor