Fix javacc plugin setup to work with jjdoc (Maven site)

* change goal from "jtree-javacc" to "jtree" AND "javacc" so that the
intermediate *.jj file is not deleted (needed for jjdoc)
* cleanup and reformat file
* rename unified folder javacc -> jtree-javacc
* configure jjdoc
Continue fixing javacc plugin setup to work with jjdoc (Maven site)

* copy the *.jj file to a flat path (because jjdoc on windows otherwise
introduces backslashes)