chore: GridListSection docs testing followup

[yihuiliao]

Fixes the following:

• section header missing borders on the left/right side
• incorrect indentation in the docs
• keyboard navigating causes items to end up behind the header

✅ 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:

Go to the docs:

1. Check if section header has borders on left/right side (more obvious in light mode)
2. Code has the correct indentation
3. When keyboard navigating, the items scroll into the view and are not behind the header. (Check GridList and ListBox)

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs