doc: move "Phases" up, "Contributing" down
    
    The "Phases" section breaks down what Pungi does in detail. Place it
    towards the top of the documentation table of contents.
    
    The "Contributing" and "Testing" sections are relevant to developers,
    not all users, so move them to the end.
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>