docs(cdk): provide guidance for a11y

[Splaktar]

Bug, feature request, or proposal:

Proposal

What is the expected behavior?

We should provide a11y guidance for CDK components as well as the Material components.

What is the current behavior?

All of the current Material components have an a11y section in their docs except for the CDK components like Table, Tree, Stepper, Drag-Drop, and Virtual Scroll (Scrolling).

What are the steps to reproduce?

https://material.angular.io/cdk/drag-drop/overview

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

We want UI library authors to publish accessible components when building with the CDK.

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

Angular 7
Angular Material 7.0.1
TypeScript 3.1.x
All browsers

Is there anything else we should know?

The current drag-drop schematic does not generate an accessible experience.

@crisbeto indicated that this article may include some helpful guidance for drag and drop a11y.