add listing style to identify non-source blocks
Update listing blocks in the Antora core docs that don't specify source with the listing style in preparation for Asciidoctor.js 2 upgrade.
- related to #600 (closed)
- related to #522 (closed)
Update listing blocks in the Antora core docs that don't specify source with the listing style in preparation for Asciidoctor.js 2 upgrade.