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:

El issue apunta a material.angular.io y al ticket #10594; empieza revisando ese ticket y la documentación actual de tablas de CDK. Añade un ejemplo documentado de rowspan que coincida con el comportamiento solicitado y, después, verifica que se renderice correctamente en el sitio de Material.

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