Improve active-TOC entry / Scrollspy behavior

[chalin]

Users are understandably confused by the current behavior, see:

• The thread starting at Update Docsy to 0.13.0 open-telemetry/opentelemetry.io#8504 (comment)
• My response, explaining the behavior: Update Docsy to 0.13.0 open-telemetry/opentelemetry.io#8504 (comment)

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).