Probably from github.com/jenkinsci/maven-site (which really needs to move to jenkins-infra anyway).
I expect that the output of maven-stapler-plugin:jelly-taglibdoc (which feeds into maven-jellydoc-plugin, which is referenced in jenkinsci/jenkins's pom.xml) is used here.
In fact, when running mvn site on jenkinsci/jenkins, the output in core/target/site/jelly-taglib-ref.html looks like what I want.
Right now, the site generation is wired to the release profile (but skipped somehow?), but we probably want to run it independently so we can do versioned output like the Javadocs.