Make documentation buildable with up-to-date tooling

We are using some pretty old artifacts around documentation. This should be revisited and updated accordingly.

One of the problems is for sure building it with newer JDKs.
The other is that during release we use Gradle to grab the whole distribution (docs included, see this repo) and that shows some additional problems.

The task in this issue is to make sure it is buildable with JDK 8 and JDK 11 and via Maven as well as Gradle build script.