Add a responsive, animated underline indicator for navigation tabs to
improve visual focus and active-state feedback.
- Introduce CSS for .nav-tabs, .nav-tabs-item and a .nav-tabs-underline
element, including transitions, positioning, and dark mode color.
- Hide default first h1 in #content to keep header layout consistent.
- Add docs/nav-tabs-underline.js to create and manage the underline
element, observe DOM mutations, and update underline position/width on
changes, resize, and when fonts load.
- Preserve last known underline position/width across re-initializations
to avoid visual jumps.
This change makes active tab state visible with smooth movement and
ensures the underline stays synchronized with dynamic content.
