doc: rework structure
    
    This is a set of change to make the landing page nicer and easier
    to navigate:
    - the default "Welcome to the documentation of …" is replaced by a something
      useful
    - "Opting-in" is changed to "using". Since rpmautospec is now supposed to
      be the default, let's just use the usual verb.
    - The structure in autorelease.rst is adjusted so that the
      first part about normal versioning and the second part about
      "traditional versioning" are at the same level (instead of the
      second part being one level down).
    - The titles are adjusted to be unique. Before we had
      "Use the %autorelease macro" and "Using the %autorelease Macro"
      and it was easy to get lost.
    - The "Examples" section level is removed and each example is prefixed
      with "Example:". This way we have one level less in the structure.
    
    Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>