Update philosophy and why

[sapphi-red]

As these pages will be outdated after Vite 8

[sapphi-red]

I've updated philosophy page, but not sure if the wording is ok.

[sapphi-red]

The why page should be updated as it will be largely outdated.
https://github.com/vitejs/rolldown-vite/blob/rolldown-vite/docs/guide/why.md