flow-dev-guide: choose a name for the guide

I'm not sure Practice is quite the right title for this new section. I'd suggest Developing Flows would be more consistent with the other sections.

No need to change anything right away - as discussed, once we have more content in the guide I think it will be easier to identify the right way to integrate it with the existing docs. What you have here is a good start begin collaboration on it.

/cc @LEEHYUKJOO

タイトルを選ぶ前に、flow-dev-guide の既存のセクションと周辺のドキュメント構成を確認してください。ガイドのコンテンツがさらに追加される中で、「Practice」と「Developing Flows」を比較し、合意された名前を文書化して、既存のドキュメントに一貫して組み込んでください。