This commit improves the user experience by adding smooth transitions
for expanding and collapsing tree node items and documentation sections.
The introduction of these animations makes the interface feel more
dynamic and responsive to user interactions.
Key changes:
- Implement a new `ExpandCollapseTransition` component to wrap UI
elements requiring expand/collapse animations.
- Utiliz the `ExpandCollapseTransition` in tree view nodes and
documentation sections to animate visibility changes.
- Refactor CSS to remove obsolete transition mixins, leveraging Vue's
transition system for consistency and maintainability.
