Improvements to Change process documentation
    
    This updates the Change announcement SOP to be in-line with
    the current practice followed by @amoloney (in the absence of
    clearer guidance from FESCo for the Discourse changes, so far).
    It adds a new SOP for dropping Changes (which is different from
    deferring them), and extends the PgM Guide Changes page and the
    top-level Changes SOP page to link to all the individual SOPs
    (a couple were missing in each case) and also explain how a
    Change's lifecycle is ultimately completed (in the Release Day
    SOP).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>