Change examples of Community Publishing Platform to explicit list
    
    This edit addresses @till's concern that the written example of social
    media accounts was too broad and should be specific. So, this commit
    basically turns a publicly-editable wiki page into an official policy at
    the Council level:
    
        https://fedoraproject.org/wiki/Marketing_social_networks#Official_Accounts
    
    If this commit is merged, that section of the wiki should be deleted and
    pointed to the published policy instead.
    
    * https://pagure.io/Fedora-Council/council-docs/pull-request/67#c-1627a97b5b362d2104afa618ca7fd866325df87a-26
    * https://pagure.io/Fedora-Council/council-docs/pull-request/67#c-1627a97b5b362d2104afa618ca7fd866325df87a-28
    
    Signed-off-by: Justin W. Flory <git@jwf.io>