Replaces the O(N) getBoundingClientRect polling loop in the docs scroll spy with an IntersectionObserver. This eliminates potential layout thrashing and reduces main thread CPU usage during scrolling.
- Replaces `window.addEventListener("scroll")` with `IntersectionObserver`.
- Maintains active section state using `passedHeadings` set.
- Updates `setupScrollSpy` and `clearScrollSpy` to manage the observer.
- Preserves existing behavior where the active section is the last one that has passed the top offset (96px). Note: Activation now occurs when the header *fully* exits the top offset area, which is a minor behavior change (delayed by header height) but improves performance and stability.
Co-authored-by: PR0M3TH3AN <[email protected]>
