Added a11y docs for list and stack

[dpersing]

WHY are these changes introduced?

Adds accessibility information for the list and stack components, to appear in polaris-react docs and in the style guide.

See the draft Google doc for editing history for these changes and updates for other components and pages.

WHAT is this pull request doing?

• Adds accessibility documentation for the list component (web, iOS, and Android)
• Adds accessibility documentation for the stack component (web only)
• Adds an entry to UNRELEASED.md

How to 🎩

1. Check out master from polaris-styleguide.
2. In another Terminal tab or window, check out this branch from polaris-react and run the instructions for testing in a consuming project.
3. In the polaris-styleguide tab, run dev up && dev start.
4. View changes after examples and props:

• https://polaris.myshopify.io/components/lists-and-tables/list (web, Android, iOS)
• https://polaris.shopify.com/components/structure/stack (web only)

Screenshots

List

Stack

Android

iOS

[sadiesaurus]

Content looks good! Just one minor suggestion.