Workshop: fix broken Sphinx cross-references.
    
    Many of the workshop pages links are directing to URLs that end with
    ".rst" instead of ".html", as a result, these links are broken.
    
    This commit introduces explicit targets and references to ensure that
    the pages are correctly linked.
    
    Signed-off-by: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>