We just renamed the repository to nar-maven-plugin

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Redirect the old URL to the new one

We renamed the repository, therefore
http://maven-nar.github.io/maven-nar-plugin is no longer automatically
populated by the contents of the gh-pages branch of our repository...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>