docs: update dynamic button examples to use AriaAlert

[khiga8]

Closes https://github.com/github/accessibility-audits/issues/10017

Changelog

Changed

• Added AriaAlert to ensure dynamic banner content is announced.
• Move and rename announcement banner example to improve discoverability. I was looking for guidance on how to do announcements for React banner. From the main Primer Banner page, no storybook examples are available for announcements. After going directly to the storybook page, I found a relevant example. I think that renaming this and moving it to the features folder will help improve discoverability for this common usecase.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2cce5f8

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[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	104.22 KB (0%)
packages/react/dist/browser.umd.js	104.58 KB (0%)

[joshblack]

Looks great! We could also include these in our e2e tests for vrt/axe too if that'd be helpful by updating: https://github.com/primer/react/blob/main/e2e/components/Banner.test.ts#L6

[joshblack]

For docs updates should be good with no changeset! I can add a skip changeset label if that would help 👀

[khiga8]

Welp, looks like AriaAlert is broken upon testing.