docs: document Components

Elaborated the Component class

docs/site/Components.md

@@ -0,0 +1,52 @@
+---
+lang: en
+title: 'Components'
+keywords: LoopBack 4.0, LoopBack 4
+sidebar: lb4_sidebar
+permalink: /doc/en/lb4/Components.html
+---
+
+## Overview
+
+Components play an important part in the extensibility of LoopBack 4. A
+Component makes it easy for independent developers to contribute additional
+features to LoopBack. Components serve as a vehicle to group extension
+contributions such as [Context Bindings](Context.md) and various artifacts to
+allow easier extensibility of your Application.
+
+A typical LoopBack component is an [npm](https://www.npmjs.com) package
+exporting a Component class which can be added to your application.
+
+Apart from its own properties, `Component` class can have the following
+properties:
+
+- `controllers` - An array of [controller](Controllers.md) classes.
+- `providers` - A map of providers to be bound to the application
+  [context](Context.md).
+- `classes` - A map of TypeScipt classes to be bound to the application context.
+- `servers` - A map of name/class pairs for [servers](Server.md).
+- `lifeCycleObservers` - An array of [life cycle observers](Life-cycle.md).
+- `bindings` - An array of [bindings](Bindings.md) to be aded to the application
+  context.
+
+These properties contribute to the application to add additional features and
+capabilities.
+
+LoopBack 4 was built with extensibility in mind and this includes Components,
+which can be allowed to contribute additional artifacts by adding a Mixin to
+your Application class. This doesn't change how a Component is registered
+(`app.component()`) but it enables the Component to contribute additional
+artifacts. For example:
+
+- [Repositories](Repositories.md) can be contributed by a Component by adding
+  `RepositoryMixin` from `@loopback/repository` to your Application
+- [Booters](Booting-an-Application.md#booters) can be contributed by a Component
+  by adding `BootMixin` from `@loopback/boot` to your Application
+
+**Note:** Always check a component's instructions to see if it requires the use
+of a Mixin. A Mixin may automatically register a Component, saving you the
+trouble of having to do so manually. Again it's best to check the documentation
+for the given Component / Mixin.
+
+See [Using components](Using-components.md) and
+[Creating components](Creating-components.md) for more information.

docs/site/Concepts.md

@@ -63,6 +63,5 @@ LoopBack 4 introduces some new concepts that are important to understand:
 - [**Decorator**](Decorators.md): The pattern used to annotate or modify your
   class declarations and their members with metadata.
 
-- **Component**: A package that bundles one or more Loopback extensions.
-  - See [Using components](Using-components.md) and
-    [Creating components](Creating-components.md) for more information.
+- [**Component**](Components.md): A package that bundles one or more LoopBack
+  extensions.

docs/site/sidebars/lb4_sidebar.yml

@@ -141,6 +141,10 @@ children:
     url: Controllers.html
     output: 'web, pdf'
 
+  - title: 'Components'
+    url: Components.html
+    output: 'web, pdf'
+
   - title: 'DataSources'
     url: DataSources.html
     output: 'web, pdf'