Add GitHub CLI installation instructions to OS-specific getting started guides

[bhouston]

Add GitHub CLI installation instructions to OS-specific getting started guides

Problem

The current OS-specific getting started guides (Windows, macOS, Linux) do not include instructions for installing the GitHub CLI (gh), which is an important tool for contributing to the project.

Proposed Solution

Update all OS-specific getting started guides to include clear instructions for installing the GitHub CLI.

Implementation Details

1. Add GitHub CLI installation instructions to:
   • Windows getting started guide
   • macOS getting started guide
   • Linux getting started guide
2. Include verification steps (gh --version)
3. Provide brief explanation of why GitHub CLI is useful for contributors

Benefits

• Ensures new contributors have all necessary tools from the start
• Streamlines the contribution process
• Provides consistent guidance across all platforms

Acceptance Criteria

• GitHub CLI installation instructions added to Windows guide
• GitHub CLI installation instructions added to macOS guide
• GitHub CLI installation instructions added to Linux guide
• Instructions are clear, concise, and platform-appropriate
• Documentation builds and deploys successfully