Improve active-TOC entry / Scrollspy behavior

[chalin]

Users are understandably confused by the current behavior, see:

• The thread starting at https://github.com/open-telemetry/opentelemetry.io/pull/8504#issuecomment-3573331429
• My response, explaining the behavior: https://github.com/open-telemetry/opentelemetry.io/pull/8504#issuecomment-3575990186

I'm wondering if the smooth scrolling option helps with this. And if so, whether we can fix the feature-interactions it has (see docs, which explains why it's disabled).