refactor (MatCalendarHeader): separate section own demo page

Author: Tobias Schweizer

src/demo-app/datepicker/datepicker-demo.html

@@ -38,6 +38,8 @@ <h2>Options</h2>
     <mat-datepicker #startAtPicker [touchUi]="touch" [disabled]="datepickerDisabled"></mat-datepicker>
   </mat-form-field>
 </p>
+
+<h2>Datepicker with custom header</h2>
 <p>
   <mat-form-field>
     <mat-label>Custom calendar header</mat-label>