Describe the problem

First of all, thank you for Redoc — I use it frequently and it is very useful.

One UX gap I keep noticing is the left sidebar nested menu behavior:
expand/collapse is currently abrupt (no transition), which feels less modern than the rest of the UI.

Current behavior

When toggling nested items in the left sidebar, the submenu appears/disappears immediately.

Expected behavior

Submenu expand/collapse should animate smoothly (height-based accordion transition), while preserving current behavior:

• active state handling
• scroll syncing / auto-follow behavior
• keyboard interaction and ARIA semantics
• mobile and desktop layout stability
• reduced motion support (prefers-reduced-motion)

Why this matters

The sidebar is a primary navigation surface in Redoc. Smoother transitions improve perceived quality and readability, especially in large specs.

Additional context

I already prepared a small, focused implementation and can open a PR if this direction is acceptable.