https://discuss.asciidoctor.org/have-asciidoctorj-output-html-to-be-include-in-jekyll-generated-site-tp8353p8357.html
It would be a lot of work, but you could run Jekyll using JRuby, then patch Jekyll AsciiDoc to use the AsciidoctorJ APIs to invoke Asciidoctor. When Ruby code is running JRuby, it can access Java objects.
An alternative would be to use AsciidoctorJ to generate embedded HTML (as Abel suggested), then dump that into the source directory of the Jekyll site and let Jekyll apply a template to it.
I'm curious to know what extensions are available for AsciidoctorJ that you would need to use in the site. (Unless they are very complex, they probably wouldn't be that hard to rewrite in Ruby).
> Antora is nice but feels overkill for this case.
I'd argue that your readme is already entirely too long and should be divided up into a documentation site to make it more approachable. So, in fact, this would be the right time to be considering Antora. Just look at the result that the AsciiDoc IntelliJ plugin was able to achieve in very short time:
https://intellij-asciidoc-plugin.ahus1.de/docs/users-guide/index.html That project is no bigger than yours.
-Dan