Redirect scala-sbt.org to readthedocs

[muuki88]

The docs published at http://www.scala-sbt.org/sbt-native-packager/ aren't up-to-date. Instead we should point http://sbt-native-packager.readthedocs.io/en/stable/

The tricky part will be to add the redirects for all existing pages to the new page without breaking the actual documentation generation.

[ScalaWilliam]

How do you publish them to scala-sbt.org?

This is quite important, there's two different documentations out there, potentially confusing new users.

[ScalaWilliam]

@muuki88