Move Philosophy into Book

The book does a great job on the How. It explains well how Nushell works. But it lacks a bit on the Why. The Introduction hints on it but it seems to get lost on the surface, e.g. "modern style of development", "modern feel", "UX polish". New users tend to want to know the Why first before the How.

The Philosophy section of the Contributor Book does a great job on the Why. But it's usually read only long after the How and only by people interested in the underlying details.

I'd like to propose moving the Why before the How. This means moving the Philosophy section into the main book before any concrete "Usage" sections. It would probably make sense to put it just after the Installation section.

Contributor Book の Philosophy セクションを、メインブックの構成、特に Installation セクションと Usage セクションと比較する。メインブック内で Philosophy を Installation の後、具体的な Usage セクションの前に移動する。Why が How より前に提示され、Contributor Book での以前の配置が処理されれば完了。