DOIs for case studies

@bnicenboim suggested here: https://github.com/stan-dev/stan-dev.github.io/issues/37#issuecomment-309366022

you should think on adding DOIs to the case studies and the Stancon papers. If I want to refer to them in a paper, it's much better than to have a link (and journals usually ask for the DOIs).

We generate DOIs for the software, but we don't know how to generate DOIs for case studies. So any suggestions would be appreciated (feel free to just edit the issue rather than adding comments if there's a concrete workable suggestion).

ソフトウェア、および issue で参照されているケーススタディと Stancon 論文について、既存の DOI プロセスを確認します。DOI を割り当てるための具体的で実行可能なアプローチを決定し、何を変更する必要があるかを文書化します。そのアプローチを適用できる程度に明確になった時点で、issue は完了です。