Add feature support tracking documentation

[MaTriXy]

Summary

• Added comprehensive feature support tracking documentation (FEATURE_SUPPORT.md)
• Documents which Rive runtime features are currently supported in the Compose Multiplatform wrapper
• Identifies implementation gaps and provides prioritized recommendations for future development

Changes

• Created FEATURE_SUPPORT.md with detailed feature compatibility matrix
• Added link to feature support doc in README.md for better discoverability
• Categorized features as Fully Supported ✅, Partially Supported ⚠️, or Not Supported ❌

Documentation Structure

The feature support document covers:

• Core animation features (state machines, playback controls, artboard selection, alignment/fit)
• Data & input features (data binding, nested inputs, text runtime)
• Advanced features (events, layouts, audio)
• Loading methods (resource, byte array, URL)
• Platform-specific implementation details
• Implementation gaps with prioritized recommendations

Motivation

This documentation helps developers understand:

• What Rive features are available in the wrapper
• What features are missing compared to native SDKs
• What to prioritize for future development

This will be valuable for both contributors and users of the library to understand the current state and roadmap.

Test Plan

• Documentation builds correctly
• All links work properly
• Feature status accurately reflects current implementation