update Rust guidelines and example spec files
    
    - mention rust2rpm more prominently since it creates 99% Guidelines-compliant
      spec files
    - BuildRequires are now automatically generated and no longer have to be
      specified manually
    - include how to generate the effective License tag for subpackages containing
      statically linked Rust binaries
    - Modularity is no longer used for building on any branch, add a short note
      about the build process for Rust packages
    - crate packages will start being available with fedora 34 in addition to
      rawhide (see the F34 System-Wide Change page for details:
      https://fedoraproject.org/wiki/Changes/Rust_Crate_Packages_For_Release_Branches )
    - mention general Updates policy exceptions for source-only Rust packages
    - refreshed .spec file examples with current specs from rawhide