More a11y fixes for Storybook stories

[TylerJDev]

Closes https://github.com/github/primer/issues/3429, https://github.com/github/primer/issues/3479

Pairs with primer/design#859

Adds more context to some storybook stories.

Changelog

New

Changed

• Add icons to Flash examples, to not solely rely on color to convey meaning
• Add Heading to PageHead examples, to demonstrate real-world usage with a heading

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]

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Latest commit: e48f0b0

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[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	97.16 KB (0%)
packages/react/dist/browser.umd.js	97.4 KB (0%)

[francinelucca]

👍🏽