[Docs] Add "project layout reference" page

[dhmlau]

In LB3, there's a Project layout reference page. It would be useful to have a similar version in LB4.

Acceptance Criteria

• Using https://loopback.io/doc/en/lb3/Project-layout-reference.html as a reference, create a similar page under LB4.

[dhmlau]

FYI - Some of the info can be found: https://loopback.io/doc/en/lb4/todo-tutorial-scaffolding.html#structure