fix: (WIP) v3 submenu positioning in the docs

[LFDanLu]

Closes

✅ 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:

[rspbot]

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[snowystinger]

Tested against #7654 and while it doesn't cause flipping, it does stop it from rendering the popover completely out of view, so it's an improvement for that. I tested some other PRs which appear to have been fixed along the way and didn't find a regression with this change.

I tested in various popover stories with different window widths and heights as well as with position relative etc and didn't find any new issues.