Add :orphan: metadata to data-model and persistent-identifiers doc files

to prevent sphinx from complaining about these not being included.

docs/data-model.rst

+:orphan:
+
 .. _data-model:
 
 Data model

docs/persistent-identifiers.rst

+:orphan:
+
 .. _persistent-identifiers:
 
 Persistent identifiers
@@ -235,5 +237,3 @@ References
   <https://hal.archives-ouvertes.fr/hal-01865790v4>`_. In Proceedings of `iPRES
   2018 <https://ipres2018.org/>`_: 15th International Conference on Digital
   Preservation, Boston, MA, USA, September 2018, 9 pages.
-
-