Restore known UUID for /content/documents

Several downstream mvn projects reference the historical known value of the /content/documents node UUID, as was produced by the archetype. In particular, the reporting dashboard addon includes references in the hippo:docbase property, which is of String type (not Reference) and therefore cannot use a path-based reference in the YAML config.

Until (and unless) HCM supports path-based references to node UUIDs in String-valued properties, we should preserve the previous value of this node UUID for backwards compatibility.