Docs: Have a collapsed Table of Content (TOC)

(this is an issue for after all the meeting scheduling work, just adding it to the queue, not urgent)

Take a look at the hibernate manual:
http://docs.jboss.org/hibernate/orm/5.3/userguide/html_single/Hibernate_User_Guide.html
and in the TOC (table of content), try to find the section on "delete statements of HQL".

Now take a look at the optaplanner manual:
https://docs.optaplanner.org/latestFinal/optaplanner-docs/html_single/
and in the TOC try to find the section on "summary statistics of benchmarking".

Clearly, you feel the pain of not having that collapsed TOC.
hibernate and optaplanner both use asciidoc. Hibernate does use gradle and we maven, but that shouldn't make much of a difference.

Get acquainted with our docs:
https://github.com/kiegroup/optaplanner/tree/master/optaplanner-docs
The hibernate docs:
https://github.com/hibernate/hibernate-orm/tree/master/documentation
The asciidoctor-maven-plugin
https://github.com/asciidoctor/asciidoctor-maven-plugin
and asciidoctor itself:
https://asciidoctor.org

Mission: give us such a collapsed TOC too.

There are 2 ways to do this:

• Proposal A). Just set a flag on asciidoc. That would be perfect (no maintenance burden for us). But I don't think asciidoc supports it out of the box... Research that.
• Proposal B). Copy paste the hibernate approach. They mix in some .js and css files to get that toc.

Note: hibernate's docs care about older versions. We don't: our docs are just the version you're looking at. So we don't want that "Choose version" combobox. The Search field is welcome though.