[Docs] MySQL 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.

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

Acceptance Criteria

The docs page is coming from loopback-connector-mysql README.

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