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 👍

Inizia dalla documentazione collegata sui campi di testo di Angular Material e dal relativo esempio StackBlitz, quindi confrontali con la riproduzione minima collegata nell’issue. Verifica quali direttive e quale configurazione sono necessari per il ridimensionamento automatico; il lavoro è completo quando la documentazione e l’esempio mostrano solo il codice necessario e spiegano qualsiasi comportamento opzionale.

Scritto dal modello di indicizzazione a partire dal testo della issue.