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.

Confronta la sezione Philosophy del Contributor Book con la struttura del libro principale, in particolare le sezioni Installation e Usage. Sposta Philosophy nel libro principale dopo Installation e prima delle sezioni Usage concrete; completato quando il Why viene presentato prima del How e la vecchia posizione nel Contributor Book viene gestita.