Allow inline ControlGroup examples to reflow

[joshfarrant]

Summary

Updates the inline CheckboxGroup and RadioGroup examples to reflow on narrow viewports.

There's quite a large row gap between lines. This could be solved by adding style={{ rowGap: 'var(--base-size-8)' }} to the Stack used in the example. Let me know if that would be preferred.

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

• Closes https://github.com/github/primer/issues/4672
• Closes https://github.com/github/primer/issues/4673

Screenshots:

Before	After

Having to add an inline style to the example isn't ideal, but without it the rowGap is way too big

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Latest commit: cff4ee8

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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, just minor feedback.