[Docs] Postgresql connector docs should cover LB4 content

[dhmlau]

Originated from #4271

While the connectors can be used for both LB3 and LB4, the tutorials under the Connectors reference for LB4 are about LB3. It might cause confusion to the users.

• postgresql connector: https://loopback.io/doc/en/lb4/PostgreSQL-connector.html
• postgresql connector tutorial: https://loopback.io/doc/en/lb4/Connecting-to-PostgreSQL.html

Acceptance Criteria

For the two pages: https://loopback.io/doc/en/lb4/PostgreSQL-connector.html and https://loopback.io/doc/en/lb4/Connecting-to-PostgreSQL.html:

• Create separate sections that are specific to LB3 and LB4.
  • The idea is to keep the LB3 content as well.

[agnes512]

Same for the READNE file (page https://loopback.io/doc/en/lb4/PostgreSQL-connector.html). Most of the setups, examples are in LB3 form. Should change or add contents for LB4.