fix broken links
    
    The linkchecker tool was used to identify broken links in the generated
    documentation.  After running 'make && make serve' in one terminal,
    linkchecker is run in another:
    
        linkchecker --check-extern --user-agent="curl/7.61.1" \
                    --ignore-url="file:///antora/" \
                    --ignore-url="https://crates.io" \
                    --ignore-url="https://hosting-service/" \
                    http://0.0.0.0:8000/packaging-guidelines/
    
    One broken link remains: the Perl SIG "New Package SCM Request" link.
    This has no direct replacement (in the context of adding an 'initial-cc'
    to bug reports), as far as I can tell.  That text and link need to be
    rewritten.  In context:
    
        New Perl packages should set the
        https://lists.fedoraproject.org/admin/lists/perl-devel.lists.fedoraproject.org/[Fedora
        Perl SIG mailing list] as a member of the initial-cc list for bugzilla.
        This can be done by adding the user `+perl-sig+` to the initial CC list
        when creating the link:Package_SCM_admin_requests#New_Packages[New Package
        SCM Request].
    
    We ignore https://crates.io because it behaves badly with non-browser
    tools (refer to https://github.com/rust-lang/crates.io/issues/788 for
    details).
    
    The --user-agent option is necessary to work around issues at sites like
    drupal.org, which return a 403 when used with linkchecker's default
    User-Agent header (LinkChecker/X.Y).