Update Banner accessibility guidance

[khiga8]

What are you trying to accomplish?

This PR makes the existing Banner accessibility guidance more concise.

There was also recent confusion around why a live region announcement when a hidden Banner with a live region attribute was unhidden. This PR makes clear that hidden Banners are also subject to not work well with live regions.

List the issues that this change affects.

Fixes: https://github.com/github/accessibility/issues/6320

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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Latest commit: ff93de5

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