Update the documentation about mass-branching
    
    * The pkgdb2 script to do mass-branching has been improved to support
    updating rawhide and creating the new collection from the CLI.
    * To create the branches correctly during the mass-branching there is
    now a ``mkbranch_branching`` that needs to be used, but only for the
    mass-branching
    * Drop the section about puppet since it's all in ansible now
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>