keep local copies of dtd and use it

This commit ensures we don't rely on and external site for the docs to build.
If that site were to go away, we wouldn't be able to build docs.

Author: swills

doc/xml2rfc/reference.RFC.2119.xml

@@ -0,0 +1,44 @@
+<?xml version='1.0' encoding='UTF-8'?>
+
+<reference anchor='RFC2119'>
+
+<front>
+<title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement Levels</title>
+<author initials='S.' surname='Bradner' fullname='Scott Bradner'>
+<organization>Harvard University</organization>
+<address>
+<postal>
+<street>1350 Mass. Ave.</street>
+<street>Cambridge</street>
+<street>MA 02138</street></postal>
+<phone>- +1 617 495 3864</phone>
+<email>[email protected]</email></address></author>
+<date year='1997' month='March' />
+<area>General</area>
+<keyword>keyword</keyword>
+<abstract>
+<t>
+   In many standards track documents several words are used to signify
+   the requirements in the specification.  These words are often
+   capitalized.  This document defines these words as they should be
+   interpreted in IETF documents.  Authors who follow these guidelines
+   should incorporate this phrase near the beginning of their document:
+
+<list>
+<t>
+      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+      "OPTIONAL" in this document are to be interpreted as described in
+      RFC 2119.
+</t></list></t>
+<t>
+   Note that the force of these words is modified by the requirement
+   level of the document in which they are used.
+</t></abstract></front>
+
+<seriesInfo name='BCP' value='14' />
+<seriesInfo name='RFC' value='2119' />
+<format type='TXT' octets='4723' target='http://www.rfc-editor.org/rfc/rfc2119.txt' />
+<format type='HTML' octets='17970' target='http://xml.resource.org/public/rfc/html/rfc2119.html' />
+<format type='XML' octets='5777' target='http://xml.resource.org/public/rfc/xml/rfc2119.xml' />
+</reference>