Always show navigation outline when scrolling

### Problem description

When scrolling down through a documentation article, the navigation outline remains aligned with the top of the page content and thus scrolls off from the visible view. The user has to scroll to the top to see the navigation again before he can choose another navigation item.

### Expected behavior

The navigation should always be visible when scrolling through the documentation article. Maybe let it stick to the current item when scrolling like the navigation outline in [Apple's HIG documentation](https://developer.apple.com/ios/human-interface-guidelines/bars/search-bars/).