Improve ToC

[strogonoff]

The idea is to eliminate duplicate ToC functionality provided by the jQuery-based script, and implement following features without that script:

— Active item highlighting
— Auto-scrolling of ToC sidebar to ensure currently selected item is visible as user reads the main pane

We’d build this based on the static ToC, and avoid regenerating dynamic ToC on top of static ToC unnecessarily, which would provide a slight performance improvement.

[opoudjis]

Implemented highlighting on click in scripts.html.