Add GitHub Copilot section

## Description

GitHub Copilot is now a core part of the GitHub ecosystem and should be documented in the cheat sheet.

## Suggested Content

- Inline code suggestions
- Copilot Chat in IDE
- Copilot for Pull Requests
- Copilot CLI integration
- Privacy and data handling

## Why This Matters

Copilot is transforming how developers write code and is one of GitHub's most talked-about features.