Documentation build: Upgrade pressgang-tools from 2 to 3

We're still using pressgang-tools 2:
https://github.com/droolsjbpm/droolsjbpm-build-bootstrap/blob/master/pom.xml#L409
but some years ago pressgang-tools 3.x was build and released (by Steve Ebersole), which fixes some issues and enables some new features (such as topics in docbook):
https://github.com/pressgang/pressgang-tools

However, pressgang-tools 3.x has an important regression in combination with the maven-docbook-plugin 2.3.8, which prevents building offline, which is a blocker for us:
https://issues.jboss.org/browse/PRESSGANG-79
The interesting note is that the gradle-docbook-plugin does build offline, and that both plugins (maven and gradle) use the same jdocbook-core:
https://github.com/pressgang

Goal: Fix that jira in maven-docbook-plugin and/or pressgang-tools, so droolsjbpm can upgrade to pressgang-tools 3.x. This is probably not easy.

The reason that it doesn't build offline, is because the xslt's contain full-url imports, so it reaches out to download those, but the jdocbook-core is able to intercepts to calls and reroute them to the local resources in the pressgang-tools jar (instead of downloading them from the internet). For some reason that works (probably configured correctly) for the gradle plugin, but not for the maven plugin. It's also strange that the maven plugin does work correctly with pressgang 2 jars.

In any case, the build should work offline (and doing "mvn -o" alone doesn't suffice to prove it, because the -o is ignored for the xslt stuff), plug out the network cable to verify it builds offline.