Docs: Time to default to setup (composition api)

[sofuxro]

Vue3 has defaulted to setup for a while now ( see the reasons )

Every new developer entering the Vue ecosystem will default to setup (composition API) and learning VTU will be a pain point.
Let's change this and make <script setup> default for VTU docs.

[cexbrayat]

I think this is now complete thanks to @sofuxro work