feat: add vite doc command using VitePress

[Copilot]

This PR adds a new built-in vite doc command that integrates VitePress for documentation serving and building, following the same patterns as other built-in commands like lint, test, and build.

Usage

# Serve documentation (default VitePress dev command)
vite doc

# Build documentation for production
vite doc -- --build

# Serve with custom host
vite doc -- --host 0.0.0.0

# Pass any VitePress arguments
vite doc -- --port 3000 --open

Implementation

The implementation follows the established architecture pattern:

• Rust Core: Added Doc command variant and doc.rs module that handles VitePress execution
• JavaScript Resolver: Created doc.ts that resolves VitePress binary path with helpful error messages when VitePress is not installed
• NAPI Bindings: Updated binding layer to include doc resolver with proper error handling
• Error Types: Added DocFailed error variant for consistent error reporting

Error Handling

When VitePress is not installed, users get a helpful error message:

VitePress not found. Please install vitepress in your project:
  npm install -D vitepress

Testing

• Added comprehensive unit tests for command parsing
• End-to-end testing confirms the command successfully starts VitePress
• All existing tests continue to pass
• Error handling verified for missing dependencies

This provides a unified developer experience where documentation is handled the same way as other development tasks in the vite+ ecosystem, making it easy to serve and build documentation alongside other project tasks.

Fixes #121.

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[fengmk2]

#172