[docs] Clarify local vs managed Prisma init commands

[francemazzi]

Docs page

https://www.prisma.io/docs/getting-started/prisma-orm/quickstart/prisma-postgres

Problem

The quickstart currently shows npx prisma init --db --output ... as the initialization step.
However, --db provisions a managed Prisma Postgres database (online login + database creation), and is not the right default for users who want a fully local setup (self-managed Postgres).

Proposed change

Clarify the section by presenting two options:

• Local setup (self-managed Postgres): npx prisma init
• Managed Prisma Postgres provisioning: npx prisma init --db --output ../generated/prisma

Why this matters

This avoids confusion and helps users choose the correct setup path based on whether they want local Postgres or managed Prisma Postgres.