We use the NekoDTD tool packaged as a Cocoon Generator to generate automagically some hyperlinked DTD reference documentation. Note that these references do not reflect the modularity of our DTDs, they are primarily "user-oriented help" documentation.
The actual DTDs are located in the Forrest distribution at
main/webapp/resources/schema/
Generated documentation for Apache Forrest document-v20 ...
Generated documentation for Apache Forrest document-v13 ...