Update BreakoutBanner docs information on leading visual accessible label

[joshfarrant]

Summary

Updates the BreakoutBanner documentation to include information on the leading visual accessible label. As the leading visual shouldn't be used for decorative imagery (source) I think it's safe to say that an accessible label should always be provided.

You can view the updated docs here

Supporting resources (related issues, external links, etc):

• Closes https://github.com/github/primer/issues/3845

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

[changeset-bot]

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Latest commit: 224db61

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[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[rezrah]

Looks great!