docs: add CITATION.cff (and optionally a Zenodo DOI) so the catalog can be cited as a dataset #114

Why it matters
Game-studies courses, licensing researchers and student projects are a natural audience for a dated, sourced licence catalog. CITATION.cff makes GitHub show "Cite this repository"; connecting Zenodo mints a DOI per release, which academic users need and which lists the project in one more index.

Evidence

• No CITATION.cff or codemeta.json in the repo root. LICENSE is CC0-1.0, so archiving is unencumbered.

Suggested approach
Add CITATION.cff (type dataset, title, maintainer, repository URL, licence, date-released kept current by the release workflow). If releases are adopted, enable the Zenodo integration and add the DOI badge to the README.

Done when
GitHub shows "Cite this repository" and the file validates.