Query Document Type of documents linked to POs

[salomerohr]

From Purchase Orders, being able to query the Document Type of the attached document: e.g. https://catalogue.library.cern/backoffice/acquisition/orders/a7g30-d6j48 query what is the Document Type of the attached document? That would be to know if an order is for a Book, an Article, etc.

(That would be mainly for statistical reasons, so feel free to add it to the issue #878 if it fits better there.)