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.

Esamina le pagine della documentazione di CDK per Table, Tree, Stepper, Drag-Drop e Virtual Scroll, confrontandole con le sezioni esistenti sull'accessibilità per i componenti Material. Leggi l'articolo collegato sull'accessibilità del drag-and-drop e ispeziona lo schematic attuale di drag-drop; il lavoro è considerato completato quando sono state affrontate le linee guida CDK interessate e la lacuna di accessibilità indicata dello schematic.

Scritto dal modello di indicizzazione a partire dal testo della issue.