Docs: request-handling pipeline

[bajtos]

This is a follow-up for the spike #2318, see the discussion and PoC code for more context.

Describe request handling steps. Explain the order in which different middleware and route handlers are invoked in LB4, what extension points are provided. This content can go into a new section to Advanced topics in Sequence or we can create an entirely new page.

When #2389 is done, requests will be handled in the following steps:

1. Request-preprocessing middleware like CORS, this is not customizable yet.
2. Native LoopBack 4 routes (controller methods or route handlers).
3. External Express routers (if the request was not handled yet).
4. Static assets (if the request was not handled yet).

Related issues: #1849

[dhmlau]

Some feedback from @raymondfeng @b-admike : #1833 (comment)