These directories are created on a per-package basis for major, well-known DTDs and applications that have a large number of architecture independent files.
Common or minor DTDs, declarations, entities should be placed in /usr/share/sgml/{dtd,entities,declaration} as appropriate.
One possibility would look like this:
| /usr/share/sgml/ |
|
The sgml-data package also creates a number of symlink-based directories here, via the common practice of mapping public identifiers to system identifiers.