sphinx/conf.py: Add redirect for devel/swh-model/persistent-identifiers
That file was renamed to devel/swh-model/swhids in swh-model!341 so ensure to not break previous URL by redirecting to the new one.
Staging instance, all changes can be removed at any time
That file was renamed to devel/swh-model/swhids in swh-model!341 so ensure to not break previous URL by redirecting to the new one.