fix(s2-docs): auto-scroll and expand section for group selected nav link

[domhhv]

Closes #9663

✅ 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:

Current behaviour (prod):

Screen.Recording.2026-02-15.at.19.36.53.mov

Ideal behaviour (this changeset):

Screen.Recording.2026-02-15.at.19.38.06.mov

Summary of changes needed for this in packages/dev/s2-docs/src/Nav.tsx:

1. Auto-expand disclosure when the current page's group, not only section, matches the nav section name
2. Auto-scroll the selected SideNavLink into view on mount
3. Add scrollMarginTop to prevent the link from being hidden behind the fixed header
4. Add type annotation to linkRef to pass TypeScript checks

🧢 Your Project:

https://github.com/domhhv/habitrack

[snowystinger]

Thanks for the PR, just a minor note

[snowystinger]

I think we'll want to use our scrollIntoViewport from @react-aria/utils

This way it'll only scroll into view if it's currently out of view and it'll fix some issues across browsers.

[snowystinger]

You'll also need to sign the CLA https://github.com/adobe/react-spectrum/blob/main/CONTRIBUTING.md#contributor-license-agreement