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:

The issue points to material.angular.io and ticket #10594; start by reviewing that ticket and the current CDK table documentation. Add a documented rowspan example matching the requested behavior, then verify that it renders correctly in the Material site.

Written by the indexing model from the issue text.