Include groovydoc generation in the groovy portlet templates

[Steffengreiner]

Is your feature request related to a problem? Please describe.

As of now we have to manually add thegmavenplus-pluginand it's dependencies to the section of the pom.xml of newly created portlets.
It would be nice to have this automatically included in the portlet_groovy and portlet_groovy_osgi
Templates so this doesn't get lost during a remerge of the Template to the project or while creating a new portlet.

Describe the solution you'd like

1.) Add the gmavenplus-plugin with the correct goals to the section of the pom.xml. for both Templates.
2.) Add the maven-site-plugin dependency to the section of the pom.xml of the Templates
3.) Check if it works by locally running the command mvn site in the root directory.

[Steffengreiner]

This should only be tackled after this Issue addressing the removal of theparent-pom from the Templates has been done.
Afterwards the steps outlined in this Issue on the Data-model-lib repository have to be added to the Pom.xml of the portlet templates.