Add DOI to CITATION.cff

Added DOI to the citation file.

Author: PinoEire

CITATION.cff

@@ -9,4 +9,5 @@ authors:
     orcid: "https://orcid.org/0000-0002-4362-7140"
 license: "Apache-2.0"
 repository-code: "https://github.com/DFTGames/UnityLocalization"
-url: "https://github.com/DFTGames/UnityLocalization"
+url: "https://github.com/DFTGames/UnityLocalization"
+doi: 10.5281/zenodo.18244510