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

.github/workflows/ci.yml

@@ -19,8 +19,6 @@ jobs:
       TARGET_DIR: ./target
       # Emit backtraces on panics.
       RUST_BACKTRACE: 1
-      # Apparently needed to use a2x on macOS.
-      XML_CATALOG_FILES: /usr/local/etc/xml/catalog
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:

.github/workflows/release.yml

@@ -75,8 +75,6 @@ jobs:
       RUST_BACKTRACE: 1
       # Build static releases with PCRE2.
       PCRE2_SYS_STATIC: 1
-      # Apparently needed to use a2x on macOS.
-      XML_CATALOG_FILES: /usr/local/etc/xml/catalog
     strategy:
       matrix:
         build: [linux, linux-arm, macos, win-msvc, win-gnu, win32-msvc]