Meta: Use partials for reusable data about the Council
    
    This commit adopts the current best-practice for using partials in
    Antora within the Council docs, and it also adds an attributes partial
    that includes common metadata used across the Council docs. The partials
    are not yet used across the Council docs, but the attributes partial is
    imported into the headers of multiple documents.
    
    This is setting the stage to make updating things like our Discourse,
    Matrix, and other comms places easier to maintain and update.
    
    Signed-off-by: Justin W. Flory (he/him) <jwf@redhat.com>