docs(aio): add NgModule docs (#20306)

PR Close #20306

aio/content/guide/sharing-ngmodules.md

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+# Sharing Modules
+
+#### Prerequisites
+A basic understanding of the following:
+* [Feature Modules](guide/feature-modules).
+* [JavaScript Modules vs. NgModules](guide/ngmodule-vs-jsmodule).
+* [Frequently Used Modules](guide/frequent-ngmodules).
+* [Routing and Navigation](guide/router).
+* [Lazy loading modules](guide/lazy-loading-ngmodules).
+
+
+<!--* Components (#TBD) We don’t have a page just on the concept of components, but I think one would be helpful for beginners.-->
+
+<hr>
+
+Creating shared modules allows you to organize and streamline your code. You can put commonly
+used directives, pipes, and components into one module and then import just that module wherever
+you need it in other parts of your app.
+
+Consider the following module from an imaginary app:
+
+
+```typescript
+import { NgModule }            from '@angular/core';
+import { CommonModule }        from '@angular/common';
+import { FormsModule }         from '@angular/forms';
+import { OrdersPipe }         from './orders.pipe';
+import { NewItemDirective }  from './newitem.directive';
+@NgModule({
+ imports:      [ CommonModule ],
+ declarations: [ OrdersPipe, NewItemDirective ],
+ exports:      [ OrdersPipe, NewItemDirective,
+                 CommonModule, FormsModule ]
+})
+export class SharedModule { }
+```
+
+Note the following:
+
+* It imports the `CommonModule` because the module's component needs common directives.
+* It declares and exports the utility pipe, directive, and component classes.
+* It re-exports the `CommonModule` and `FormsModule`.
+
+By re-exporting `CommonModule` and `FormsModule`, any other module that imports this
+`SharedModule`, gets access to directives like `NgIf` and `NgFor` from `CommonModule`
+and can bind to component properties with `[(ngModel)]`, a directive in the `FormsModule`.
+
+Even though the components declared by `SharedModule` might not bind
+with `[(ngModel)]` and there may be no need for `SharedModule`
+to import `FormsModule`, `SharedModule` can still export
+`FormsModule` without listing it among its `imports`. This
+way, you can give other modules access to `FormsModule` without
+having to import it directly into the `@NgModule` decorator.
+
+### Using components vs services from other modules.
+
+There is an important distinction between using another module's component and
+using a service from another module.. Import modules when you want to use
+directives, pipes, and components. Importing a module with services means that you will have a new instance of that service, which typically is not what you need, (typically one wants to reuse an existing service.) Use module imports to control service instantiation. 
+
+The most common way to get a hold of sharedservices is through Angular
+[dependency injection](guide/dependency-injection), rather than through the module system (importing a module will result in a new service instance, which is not a typical usage).
+
+To read about sharing services, see [Providers](guide/providers).
+
+
+<hr />
+
+## More on NgModules
+
+You may also be interested in the following:
+* [Providers](guide/providers).
+* [Types of Modules](guide/module-types).
+
+
+