add an index listing all licenses at page start

issue:
When a project uses multiple licenses, and they are their content is rendered,
it is difficult to even notice that there are multiple ones, because one possibly
has to scroll down multiple screens to see the start of the second one.

proposed solution:
When a project uses multiple licenses, and license rendering is not set to linkOnly,
then render an index at the start of the page, containing links to the section
anchors of the licenses.

patch:
implements the proposed solution, though it may have to be heavily altered,
as i never wrote something for a maven (report) plugin.