@Override
    public Document createFieldValue() {
        return new Document(this, (DocumentId) null);
    }
Creates a new document type and registers it with the document type manager.
Implicitly registers this with the document type manager.
The document type id will be generated as a hash from the document type name.

@param name       The name of the new document type
@param contentStructType The type of the content struct
document/src/main/java/com/yahoo/document/DocumentType.java