Antora Xref Extension

The Antora Xref Extension augments Antora’s support for xref macros with the following features:

• Validate that the target file contains an anchor that matches the fragment.
• Use default link text from the title or reftext of the target.
• Stub links that don’t yet exist
• Produce warnings if link text is unnecessary

See https://github.com/spring-io/antora-xref-extension/ for more details and usage instructions.