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.

Table、Tree、Stepper、Drag-Drop、Virtual Scroll の CDK ドキュメントページを確認し、Material コンポーネントに関する既存のアクセシビリティセクションと比較してください。リンクされている drag-and-drop のアクセシビリティ記事を読み、現在の drag-drop schematic を調査してください。影響を受ける CDK ガイダンスと、記載されている schematic のアクセシビリティ上のギャップに対応できれば完了です。

索引モデルが issue の本文から書いたものです。