Simplified things by including books.xslt, download.xslt and security.xslt into article.xslt, and always using article.xslt to generate HTMLs. While here, moved versions.xml from common dependencies to article dependencies. Fixed menu in 404.html by applying templates from menu.xslt, and fixed its dependency on DTD.

<!--
  Copyright (C) Igor Sysoev
  Copyright (C) Nginx, Inc.
  -->

<!ELEMENT  i18n         (text+) >

<!ELEMENT  text         (item+) >
<!ATTLIST  text
           lang         CDATA #REQUIRED
>

<!ELEMENT  item         (#PCDATA) >
<!ATTLIST  item
           id           CDATA #REQUIRED
>