docs: add comprehensive guide to Flutter animations

[AbhishekDoshi26]

Description

This PR adds a highly technical, objective reference guide to Flutter animations under the flutter-concepts section.

The document strips away conversational fluff and focuses purely on the underlying architecture and APIs required for high-performance motion in Flutter.

Highlights:

• Added animations.mdx (sidebar order 6).
• Implicit Animations: Covers ImplicitlyAnimatedWidget internals and non-linear interpolation (Curves).
• Explicit Animations: Details AnimationController, TickerProvider (vsync), CurvedAnimation, and Tween mapping. Includes a Mermaid flow diagram visualizing the architecture.
• Physics-based Simulations: Provides examples for SpringSimulation and controller.animateWith().
• Performance Considerations: Outlines best practices for avoiding layout passes by animating opacity and transform properties directly on the GPU.
• Linting: The document strictly complies with all Vale rules (no first-person pronouns, sentence case headings, no exclamation points).