Spurious Javadoc VM-initialization errors from offline-link generation for reactor dependencies

The maven-javadoc-plugin has detectOfflineLinks enabled by default, which makes it generate -linkoffline cross-links to the Javadoc of reactor-dependency modules. When such a dependency's apidocs have not been generated yet (for example xwiki-commons-legacy-velocity depends on xwiki-commons-legacy-velocity-tools), the plugin forks a new Maven process to generate them, and that fork fails to initialize its JVM. This produces a cascade of confusing errors during the release/quality Javadoc JAR generation:

The goal '...:javadoc' has not been previously called for the module ... Trying to invoke it...
Error occurred during initialization of VM ...
Creating fake javadoc directory ...
Error fetching link: .../apidocs. Ignored it.

We don't publish an aggregated Javadoc site (each module's Javadoc JAR is deployed independently and cross-module navigation goes through the custom project URL), so these offline links serve no purpose. Fix by disabling detectOfflineLinks in the shared plugin configuration.