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.

Comparer la section Philosophy du Contributor Book avec la structure du livre principal, en particulier les sections Installation et Usage. Déplacer Philosophy dans le livre principal après Installation et avant les sections Usage concrètes ; terminé lorsque le Why est présenté avant le How et que l’ancien emplacement dans le Contributor Book est traité.