allow the pdf generation to be specified as a report in project.xml

actually you can only set the generation of the pdf file during site:generate using a pre/post goal in maven.xml (usually people need to add a site:generate postGoal).

It will be nice to avoid a custom maven.xml allowing the pdf generation to be specified in project.xml reports:
<reports>
<report>pdf</report>
<reports>

The attached patch adds the required register/deregister/report goals and updates the documentation.