docs(scroll): add iOS 18+ onScrollGeometryChange and scrollPosition APIs

[elio-Wang]

Add onScrollGeometryChange and scrollPosition() coverage, restructure
Scroll Position Tracking to present the modern API first, and demote
the GeometryReader + PreferenceKey approach to a legacy fallback note.

Closes #68

[AvdLee]

Thanks for adding these modern scroll APIs — this addresses an important gap where agents can otherwise fall back to older geometry-preference patterns.

I pushed a follow-up commit to keep that intent while correcting and tightening the guidance:

• Changed the iOS 18 scrollPosition(_:) examples to bind a ScrollPosition, which is the type required by the unlabeled overload.
• Added .scrollTargetLayout() for identity-based scrolling and showed position.scrollTo(id:) for programmatic navigation.
• Added the distinct iOS 17 scrollPosition(id:) form with an optional ID binding so agents do not mix the two overloads.
• Corrected the offset-threshold direction and accounted for the top content inset.
• Clarified that extracting a CGFloat offset still changes every scrolling frame, while deriving a Bool limits actions to threshold crossings.
• Fixed the header fallback so it becomes visible again when returning above the threshold.
• Consolidated the duplicated iOS 18 sections into one source of truth and described the geometry/preference technique as pre-iOS 18 compatibility rather than a deprecated API.

I type-checked the revised iOS 17 and iOS 18 API examples against the respective simulator SDK targets. Both compile successfully.