Add "contribute" and "coding style" pages
    
    This is pretty much a copy & paste from the original one, with basically
    a few changes:
    
    - pointing some links to pagure instead of fedorahosted;
    - mentioned the commit-template *after* the part where you clone the
    repo;
    - changed the commit used as example for one that actually matches our
    commit template.
    
    The coding style page has been added together to this PR as it's
    referenced by the contribute page.
    
    There are also some FIXME added to the contribute page as would be nice
    to add back some links to some pages that were not yet migrated.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Co-author: Jakub Hrozek <jhrozek@redhat.com>