DOC: Add comprehensive BUILDGUIDE.md with cross-platform build instructions and VS Code setup

[Copilot]

Work Item / Issue Reference

GitHub Issue: #145

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Summary

This pull request adds a comprehensive build guide for contributors to the mssql-python project. The guide provides detailed instructions for setting up the development environment, building native bindings, packaging the project as a Python wheel, and running tests.

Documentation Enhancements:

• New BUILDGUIDE.md file that includes:
  • Getting Started section with both command-line and Visual Studio Code approaches for forking and cloning
    • Command-line instructions for traditional Git workflows
    • VS Code-specific instructions including GitHub extension installation and integrated Git workflows
  • Prerequisites for development, including Python 3.10+, pybind11, and CMake
  • Platform-specific setup instructions for Windows, macOS, and Linux
    • Windows setup includes alternative VS Code configuration option
  • Steps for building native bindings using platform-specific scripts with correct directory paths
  • Instructions for building Python wheel packages after compiling native bindings
  • Enhanced test running guidance with proper directory navigation and environment variable setup
  • Comprehensive database setup section with detailed platform-specific commands for Windows, macOS, and Linux using go-sqlcmd for containerized SQL Server instances
  • Overview of project directory structure
  • Comprehensive troubleshooting section

Key Fixes Applied:

• Corrected all directory paths from pybind/ to mssql_python/pybind/ to match actual repository structure
• Fixed broken internal links, particularly the reference to the pybind README
• Added clear navigation instructions for running tests from the correct directory
• Enhanced Windows setup with VS Code alternative configuration
• Added comprehensive VS Code workflow instructions for contributors who prefer IDE-based development
• Added complete platform-specific database setup commands for all three major platforms (Windows, macOS, Linux) with detailed installation steps for Docker and sqlcmd

The guide now provides a complete contributor onboarding experience with accurate paths, comprehensive setup options, support for both command-line and VS Code workflows, and detailed database setup instructions for all platforms.

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This pull request was created and enhanced based on feedback to ensure all links work correctly, paths match the actual repository structure, includes modern development workflows using Visual Studio Code, and provides complete database setup instructions for Windows, macOS, and Linux.

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