When choosing Spring REST Docs, the Spring Initializr configures the asciidoctor-maven-plugin for us.
However, the current version 1.5.3 does not add the highlight JS files to the outputted HMTL documentation.
When upgrading to the latest 1.5.8 version, it does work.
Problem was also reported here on SO: https://stackoverflow.com/questions/52669916/asciidoctor-does-not-highlight-source-code-by-highlightjs