Set up redirects for the Maven Plugin documentation

[wilkinsona]

Once 2.3.0.RELEASE has shipped, some current documentation links to the Maven Plugin will be broken as there's only a single page now. We should set up some redirects to point to the equivalent anchors in the new documentation.

[snicoll]

Redirects are in place, we'll enable that once 2.3.0 is actually released and the current symlink switches to 2.3