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.

So sánh phần Philosophy trong Contributor Book với cấu trúc của sách chính, đặc biệt là các phần Installation và Usage. Di chuyển Philosophy vào sách chính, sau Installation và trước các phần Usage cụ thể; hoàn tất khi Why được trình bày trước How và vị trí cũ trong Contributor Book được xử lý.