[a11y] Add empty state component a11y docs

[sadiesaurus]

WHY are these changes introduced?

Adds accessibility guidance for the empty state component, to appear in polaris-react docs and in the style guide.

WHAT is this pull request doing?

• Adds accessibility documentation for the empty state component (web only)
• Adds an entry to UNRELEASED.md

How to 🎩

1. Check out master from polaris-styleguide to get the changes that support the accessibility section.
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/actions/account-connection (web only)
• https://polaris.myshopify.io/components/actions/setting-toggle (web only)

Screenshots

FYI @dpersing ❤️

[github-actions]

No changes

💦 Potential splash zone of changes introduced to src/**/*.tsx in this pull request:

No significant changes to src/**/*.tsx were detected.

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[dpersing]

🎉 Go ahead and squash and merge, @sadiesaurus !