docs: add more examples for Postgresql and other dialects

I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

I'm starting with Ent and I would like to have examples to copy in docs (https://entgo.io/docs/getting-started#create-your-first-entity) to initialize new client with Postgres and other dialects in addition to sqlite.

Because coming from other ORM/engines Ent can create a lot of confusion (because it's an amazingly big project) and increase the time for migration.

This copy & paste can reduce cognitive times.