Slience antora warnings.
    
    The font guidelines had a number of instances of unescaped strings like "{foo}"
    which aren't marked as literals.  Asciidoctor interprets as macro substitutions
    and complains about them.  The solution is to backwhack the first bracket, so
    something like
        *%{fontappstreams}*
    becomes
        *%\{fontappstreams}*
    
    This does not affect rendering.
    
    In general it may be better to mark these as literals instead, as is done in
    most guidelines documents.  But there are so many in the fonts document that
    escaping the few problematic ones is much more expedient.