[Docs] Update `Why Not Express` section in Crafting LoopBack 4

[virkt25]

Description / Steps to reproduce / Feature proposal

From #1362

The section https://loopback.io/doc/en/lb4/Crafting-LoopBack-4.html#why-not-express needs to be updated to reflect the fact that we have switched back to using Express -- explaining how it is used today (not for routing but only middleware ... while we still provide a sequence of actions) ... and the decision for this (access to existing middleware).

Acceptance Criteria

• Update https://loopback.io/doc/en/lb4/Crafting-LoopBack-4.html#why-not-express