[docs] SideNav jumps when main content height increases

[mj12albert]

Discovered in #2116 (comment), it's reproducible currently on the live site, just harder because the demos are the only height-expandable elements right now

It happens (easier on shorter pages) when:

1. the main content is scrolled to the bottom
2. the SideNav is still position: sticky-ed to the top, and it's own scroll position is still at the top
3. expand the demo

Screen.Recording.2025-06-18.at.11.14.14.PM.mov