fix: layout shift in s2 modal docs

[nwidynski]

Fixes layout shift in https://react-aria.adobe.com/Modal#sheet, by preventing overflow on the overlay itself. The overflow is caused by the modal transformation adding a horizontal scrollbar, which then in turn causes a vertical scrollbar on the root element, causing the layout to shift horizontally. Also adds a story so the gutter addition can be regression tested in the future.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[snowystinger]

lgtm, thanks for fixing that