docs(cdk): provide guidance for a11y

#### 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](https://medium.com/salesforce-ux/4-major-patterns-for-accessible-drag-and-drop-1d43f64ebf09) may include some helpful guidance for drag and drop a11y.

Revisa las páginas de documentación de CDK para Table, Tree, Stepper, Drag-Drop y Virtual Scroll, comparándolas con las secciones de accesibilidad existentes para los componentes de Material. Lee el artículo enlazado sobre accesibilidad de drag-and-drop e inspecciona el schematic actual de drag-drop; se considera terminado cuando se hayan abordado las directrices de CDK afectadas y la brecha de accesibilidad indicada del schematic.

Escrito por el modelo de indexación a partir del texto del issue.