[a11y] Banner a11y docs

[dpersing]

WHY are these changes introduced?

Adds accessibility guidance for the banner component, 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 banner component (web, iOS, and Android)
• 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/feedback-indicators/banner (web, iOS, and Android)

Screenshot

Web content:

[dpersing]

@danrosenthal and @sadiesaurus, would love your 👀 !

[sadiesaurus]

One teensy edit but otherwise looks fantastic to me 👍

[danrosenthal]

👍