docs: Fix installation instructions and command inconsistencies across documentation

[itsmattius]

Summary

This PR addresses several documentation inconsistencies and platform-specific issues identified in the AG-UI quickstart guides to improve developer experience and cross-platform compatibility.

Changes Made

1. Cross-Platform Protobuf Installation Instructions

Files modified:

• docs/quickstart/server.mdx

Before:

brew install protobuf

After:

# Linux (Ubuntu/Debian)
sudo apt install protobuf-compiler

# macOS
brew install protobuf

# Windows
winget install protobuf

2. Removed Unnecessary Global Turbo Installation

Files modified:

• docs/quickstart/server.mdx

Removed:

npm i turbo

Rationale: Turbo is already included as a dev dependency and available via local pnpm scripts.

3. Fixed Dojo Startup Command

Files modified:

• docs/quickstart/server.mdx

Before:

turbo run dev

After:

pnpm run dev

4. Updated Python Code for Latest OpenAI Package

Files modified:

• docs/quickstart/server.mdx

Changes:

• Updated "Implement the streaming server" Python code to be compatible with the latest version of the openai package
• Fixed API usage patterns and imports to match current OpenAI SDK
• Ensured code examples work without runtime errors

Benefits

• Cross-platform compatibility: Developers on Linux and Windows can now follow the documentation without confusion
• Reduced setup friction: Eliminates unnecessary global package installations
• Consistency: Aligns with the project's use of pnpm as the package manager
• Better developer experience: Clear, accurate instructions reduce setup time and errors
• Up-to-date code examples: Python server implementation works with current OpenAI SDK versions

Testing

• ✅ Verified protobuf installation commands work on respective platforms
• ✅ Confirmed pnpm run dev works correctly with local turbo dependency
• ✅ Tested setup instructions on Linux, macOS, and Windows environments
• ✅ Verified Python server code runs without errors with latest OpenAI package

Related Issues

Closes #205

Notes

• These changes are purely documentation improvements and don't affect the codebase functionality
• The fixes are straightforward and suitable for new contributors
• Python code updates ensure compatibility with current OpenAI SDK versions

Checklist

• Documentation updated
• Cross-platform compatibility verified
• Commands tested on multiple platforms
• Python code updated for latest OpenAI SDK
• No breaking changes introduced
• Follows project documentation standards