Major documentation clean up and preps for readthedocs

[muuki88]

Major refactoring of the native packager documentation.

• Change theme: now read-the-docs
• Removed old stuff
• Merge pages
• Added introduction

To build the theme locally you must install the read-the-docs theme

pip install sphinx_rtd_theme

cc @metasim @fiadliel @kardapoltsev

@eed3si9n @dwijnand can we redirect http://www.scala-sbt.org/sbt-native-packager/ to the read the docs manual?