add file to provide javadoc for utils-groovy project

a java file is needed for the maven project to produce javadoc, which in turn is needed to release to mavencentral; this adds such a file to the groovy project

when we (previously) built as `<packaging>bundle</packaging>` this didn't seem to be a problem, but now that we are making `jar` it is

the `maven-javadoc-plugin` only recognises java files.  there are some projects trying to do the same for groovydoc but feels high risk to introduce that, esp as we are phasing out the groovy ... hence this class

note the web site scripts make the groovy documentation so this only really impacts IDE-based javadoc for the handful of groovy classes