Flesh out the documentation to be stand-alone.

We now have the complete instructions in this README without any references to
the previous version so that we can easily switch over to this model of
deployment.
Move the Jsonnet configs for Ambari to new folder.
Point users to Ambari's new home.
Remove the original CentOS 6 deployment.

We now have a more modular and multi-OS/deployment compatible version in a new
directory, ../ambari, that folks should be using and migrating to.
Point users to the new Ambari dir from parent dir.
Link to the app dir rather than README.md itself.

By sending users to the directory, they will still see the README contents
automatically, but will also have a file browser at their disposal.

By linking directly to the README itself, we were previously just showing the
users the text but without the very useful file browser as well, which can be
used to explore the directory structure, config scripts, etc.