docs: move API references to tsdoc

As part of our adoption of the four-quadrant documentation system (see #5549 and #5718), we should move API reference material from markdown files in `site/docs` folder to tsdoc comments in the source code.

This story is a part of #5113 Documentation improvements 2020Q3.

## Acceptance criteria

List of places to modify:

- [ ] https://loopback.io/doc/en/lb4/Server.html#rest-options

- [ ] The page "Model" contains reference-like content for model/property metadata mixed with guide-like content. We should rework the content into proper guides (recipes for achieving specific outcomes) and move reference description of different metadata options into tsdocs.
- [ ] https://loopback.io/doc/en/lb4/Model.html#supported-entries-of-model-definition
- [ ] https://loopback.io/doc/en/lb4/Model.html#unsupported-entries
- [ ] https://loopback.io/doc/en/lb4/Model.html#property-decorator
- [ ] https://loopback.io/doc/en/lb4/Model.html#id-properties
- [ ] https://loopback.io/doc/en/lb4/Model.html#data-mapping-properties
- [ ] https://loopback.io/doc/en/lb4/Model.html#supported-json-keywords

- [ ] http://loopback.io/doc/en/lb4/Creating-crud-rest-apis.html#model-configuration-options

- [ ] https://loopback.io/doc/en/lb4/Using-database-transactions.html#isolation-levels

- [ ] The page "Interceptors" contains reference-like content with code snippets showing the classes & interfaces including tsdoc. Let's find a better way how to present this information, leverage existing API doc pages and avoid duplication of content.
- [ ] https://loopback.io/doc/en/lb4/Interceptors.html#invocation-context
- [ ] https://loopback.io/doc/en/lb4/Interceptors.html#source-for-an-invocation

- [ ] The page "Life cycle" contains reference-like content with code snippets the classes & interfaces including tsdoc. Let's find a better way how to present this information, leverage existing API doc pages and avoid duplication of content.
- [ ] https://loopback.io/doc/en/lb4/Life-cycle.html#graceful-shutdown
- [ ] https://loopback.io/doc/en/lb4/Life-cycle.html#the-lifecycleobserver-interface
- [ ] https://loopback.io/doc/en/lb4/Life-cycle.html#observer-groups (`export type LifeCycleObserverOptions`)

- [ ] The pages grouped under [Decorators](https://loopback.io/doc/en/lb4/Decorators_repository.html) are reference guides. Let's move their content into tsdocs and update "Decorators" page to act as a sign post linking to the API docs. For each page we have now, create a section in "Decorators" page and list the relevant decorators there.

- [ ] [OpenAPI decorators](https://loopback.io/doc/en/lb4/Decorators_openapi.html)
- [ ] [Dependency Injection decorator](https://loopback.io/doc/en/lb4/Decorators_inject.html)
- [ ] [Authentication decorator](https://loopback.io/doc/en/lb4/Decorators_authenticate.html)
- [ ] [Service decorator](https://loopback.io/doc/en/lb4/Decorators_service.html)
- [ ] [Repository decorators](https://loopback.io/doc/en/lb4/Decorators_repository.html)

- [ ] The page "Context" contains reference-like content with code snippets the classes & interfaces including tsdoc. Let's find a better way how to present this information, leverage existing API doc pages and avoid duplication of content.
- [ ] https://loopback.io/doc/en/lb4/Context.html#context-events
- [ ] https://loopback.io/doc/en/lb4/Context.html#context-observers

- [ ] https://loopback.io/doc/en/lb4/Binding.html#binding-events
- [ ] https://loopback.io/doc/en/lb4/Dependency-injection.html#additional-inject-or-sugar-decorators -- leverage the new "Decorators" page

Start by reviewing the listed pages in site/docs and the related context from #5549 and #5718. Compare the reference-like sections with existing API documentation, especially for Model, Interceptors, Life cycle, Context, and Decorators. Done means the listed reference material is represented in tsdoc, duplicated snippets are addressed, and the Decorators page links to the relevant API documentation.

Written by the indexing model from the issue text.