The Codelibrary SGML DTD was written in order to provide a framework for the libpostal docs. It's based around markup aimed directly at code libraries, so there's very well-targetted proper structural markup for all the elements that tend to come up (at least, that we've noticed so far!) in documenting a code library.
There isn't much in the way of documentation on it. However, the libpostal docs show the formatted output, as well as having the SGML input file that generated them, which will give you a starting point. Also, the distribution tarball for the Codelibrary DTD itself has a very cut-down version of the libpostal docs in its examples/ directory, with just a few data structures and functions mentioned; this is mainly to provide a quick and easy example of the usage of the DTD.
The current release is version 2.0. This brings a number of changes from the 1.0 release, including some significant simplification and removing some libpostal-specific stuff from the DTD. The various output renderings should be rather a bit nicer, too. Of course, there's lots still to be done.
To install it, just download, untar, and run 'make install'. You may wish to customize the install location. For instance, if you don't want to install it system-wide, but only in your $HOME/sgml, type make INSTDIR=$HOME/sgml install. The default install location is under /usr/local/share/sgml/, which means the actual files will be under /usr/local/share/sgml/codelibrary/2.0/. This allows you to have multiple versions installed without any extra work.
This information is pulled from the README file in the distribution
Building all these transformations uses a LOT of software packages.
If you need the older version, grab version 1.0.