Example for Rowspan attribute for cdk table

#### Bug, feature request, or proposal:
Request, creating a request as suggested in ticket #10594 by @CDDelta

#### What is the expected behavior?
Not able to implement rowspan attribute for cdk table. would be great if any example is present on material.angular.io.

#### What is the current behavior?
No example is present for rowspan attribute for cdk table.

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

#### What is the use-case or motivation for changing an existing behavior?
C

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
All

#### Is there anything else we should know?
expected example:

La issue rimanda a material.angular.io e al ticket #10594; inizia esaminando quel ticket e la documentazione attuale delle tabelle CDK. Aggiungi un esempio documentato di rowspan che corrisponda al comportamento richiesto, quindi verifica che venga visualizzato correttamente nel sito Material.

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