docs(cdk-stepper): add guide to create stepper (angular#14710)

* docs(cdk-stepper): add guide to create stepper

* Added guide how to create a custom stepper using the CDK
* Added a material example to demonstrate the custom CDK stepper
* Made steps from CdkStepper public as it is necessary to build
a custom stepper

* Review comments

Author: Michael Hoffmann

src/cdk/stepper/stepper.ts

@@ -287,7 +287,7 @@ export class CdkStepper implements AfterViewInit, OnDestroy {
       if (this._selectedIndex != newIndex &&
           !this._anyControlsInvalidOrPending(newIndex) &&
           (newIndex >= this._selectedIndex || this.steps.toArray()[newIndex].editable)) {
-        this._updateSelectedItemIndex(index);
+        this._updateSelectedItemIndex(newIndex);
       }
     } else {
       this._selectedIndex = newIndex;