Docs for exposing REST API of a Model with no custom classes

[bajtos]

Documentation for the feature From model definition to REST API with no custom repository/controller classes #2036

Acceptance criteria

• Docs for application developers consuming this feature. Describe the new CLI command, how to use controller/repositories contributed by extensions, what model config options are available.

• Docs for extension developers building custom controller/repository classes. Explain what API contract the custom controller/repository class must implement, show how the component must export the classes so that they can be picked by the Booter.

[dhmlau]

Cross posting from #4296 (comment) from @raymondfeng :

Existing docs:

• https://github.com/strongloop/loopback-next/blob/master/docs/site/Creating-CRUD-REST-apis.md (README)
• https://github.com/strongloop/loopback-next/blob/master/docs/site/Rest-Crud-generator.md
• https://github.com/strongloop/loopback-next/blob/master/examples/rest-crud/README.md (README)
  I think we can convert https://github.com/strongloop/loopback-next/blob/master/docs/site/Creating-CRUD-REST-apis.md into a regular page that glues the runtime/cli/example today to tell a story.