Add a DTD for the new module docs.

Add a DTD for the new module docs.

At the moment, not everything will validate with this DTD.  This could be
for a couple reasons:

1. The DTD needs some slight adjustments.

2. The xml needs some slight adjustments.

On the other hand, some things like mod_rewrite.xml are so far from this
DTD that I don't think I will bother trying to make it validate right
now.

Submitted By: Brad Miller <brad@beldamar.com>, Joshua Slive

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93728 13f79535-47bb-0310-9956-ffa450edef68