docs: move API references to tsdoc

[bajtos]

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 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
  • Dependency Injection decorator
  • Authentication decorator
  • Service decorator
  • Repository decorators

• 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

[bajtos]

Added a placeholder estimate 40sp