Wire up tertiary testing API docs in material.angular.io

[devversion]

We need to figure out how we handle docs generation for tertiary entry-points such as cdk/testing/testbed or material/button/harness.

cc. @mmalerba @jelbourn

[Splaktar]

There is a PR for material-aio here: angular/material.angular.io#660

[mmalerba]

@andrewseguin I believe this is done now right?

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