Added authors.ent. This is v1.93 of doc/handbook/authors.sgml, with
these changes;

1. Remove '<tt>' and '</tt>'.
2. Search/replace
   
   \s-+<htmlurl url='mailto:\([^']+\)'\s-+name='[^']+'>
   
   with
   
   <email>\1</email>
   
   (there's a leading space before <email>)

Added an ENTITY line to handbook.sgml to use the new entities.