Add minimal example for cdkTextareaAutosize

#### What is the expected behavior?
The [docs](https://material.angular.io/cdk/text-field/overview) for `Automatically resizing a ` and the [stackblitz example](https://stackblitz.com/angular/aqdvjmmqomd) should only mention what's actually needed to make it work.

#### What is the current behavior?
They mention a whole lot of stuff, **_removing which_** still lets things function! So its not clear why its been put there and its misleading without an explanation.

Extra stuff in the HTML template:
```
#autosize="cdkTextareaAutosize"
```

Extra stuff in the component:
```
import {CdkTextareaAutosize} from '@angular/cdk/text-field';
import {..., NgZone, ViewChild} from '@angular/core';
import {take} from 'rxjs/operators';
...
constructor(private ngZone: NgZone) {}
@ViewChild('autosize') autosize: CdkTextareaAutosize;
triggerResize() {
// Wait for changes to be applied, then trigger textarea resize.
this.ngZone.onStable.pipe(take(1))
.subscribe(() => this.autosize.resizeToFitContent(true));
}
}
```
That's 4 extra imports, a `viewchild` binding and when I put a `console.log` statement inside either `triggerResize` or the subscribed function ... nothing happens ... meaning the code never runs! So what is the point of all that extra stuff? Maybe its actually useful for something else that the docs failed to mention?

#### What are the steps to reproduce?
This [stackblitz](https://stackblitz.com/edit/mat-list-height?embed=1&file=src/app/app.component.html) shows that nothing other than `cdkTextareaAutosize cdkAutosizeMinRows cdkAutosizeMaxRows` is needed to `Automatically resizing a <textarea>`

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular v7, Material v7 are the versions I used.

#### Is there anything else we should know?
Keep up the good work 👍

Beginne mit der verlinkten Dokumentation zu Angular Material-Textfeldern und ihrem StackBlitz-Beispiel und vergleiche sie anschließend mit der im Issue verlinkten minimalen Reproduktion. Überprüfe, welche Direktiven und welche Einrichtung für die automatische Größenanpassung erforderlich sind; abgeschlossen ist die Aufgabe, wenn die Dokumentation und das Beispiel nur den notwendigen Code zeigen und jedes optionale Verhalten erklären.

Vom Indexierungsmodell aus dem Issue-Text verfasst.