Add maven plugin documentation

This commit adds the generated site for the maven plugin alongside
the developer guide and javadoc. The maven plugin is available in
the `/maven-plugin` context.

The advanced information described in the developer guide have
been migrated to the plugin site as most the information is taken
from the code itself, which avoids duplication.

I don't know if something special needs to be done so that the ZIP is processed as it should or if it's just unzipped on the server as is. If that's the case, we're good because the zip contains now an extra `maven-plugin` directory with the content of the site.

Fixes #749