L18 missing ELRA/LREC native IDs

The `url` and `pdf` fields in L18 both point to the Anthology PDF. For prior LRECs, these both pointed to the LREC-hosted PDF, which while not without issues did permit syncing up of other metadata across the sites. It might be easier, and avoid duplication, if both ACL and LREC paper IDs were listed in the metadata. Oh, and isn't L20 just around the corner?

(also not a correction, sorry)

Start by locating the L18 metadata records and reviewing how their url and pdf fields are represented. Clarify the expected ACL and LREC native IDs from the issue discussion, then update the metadata consistently; done means the identifiers are available without duplicating the Anthology PDF references.

Rédigé par le modèle d'indexation à partir du texte de l'issue.