R004--Open Catalog usage for DocBook

Directories like the ones holding Jade's or OpenJade's declarations and the ISO entities are on top level because they are not specific to any given DTD and can be used by two or more of them.

Of course one may argue that Jade's or OpenJade's declarations contain the document type definition of what DSSSL is. But again what is important is the usage, not the formal definition, so it has no reason to go to a dsssl/ directory (which would also encourage packagers to put the stylesheets in, away from their DTD, which is not what we want).