modules: Remove Join SIG pages and put redirects in their place
    
    This commit removes the Join SIG placeholder pages here, and instead
    links out to the existing Fedora Join SIG documentation. This saves us
    time and effort of duplicating content that already exists, and it also
    makes it more visible alongside the other content that remains to be
    written (e.g. team overview and role handbook pages).
    
    Related to fedora-join/fedora-join-docs#10.
    
    Signed-off-by: Justin W. Flory (he/him) [Fedora Project] <jwf@fedoraproject.org>