docs: fix S2 docs mobile search bugs

[reidbarber]

• Fixes issue where when you initially scrolled the page down, then opened the mobile search menu, it wasn't visible.
• Only render a table of contents in mobile header if there are subsections on a page.

✅ 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]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation
• View the S2 docs

[yihuiliao]

lgtm, tested on iphone