Add GitHub Copilot section to magic files blog post

[Copilot]

This PR adds a comprehensive new section covering GitHub Copilot configuration files to the existing "GitHub's magic files" blog post, as requested in the issue.

Changes Made

Added a new "GitHub Copilot Files" section that includes:

• Custom instructions files (copilot-instructions.md) - for providing repository-specific context and coding guidelines
• Copilot ignore files (.copilotignore) - for excluding sensitive or irrelevant files from Copilot's context
• Copilot chat configuration (copilot-chat.md) - for specialized chat prompts and interactions

The new section follows the same structure and formatting as the existing content:

• Consistent table format with columns for Filename, Location, .github repo support, Description, and Docs
• Proper documentation links to GitHub's official Copilot documentation
• Explanatory text about how these files help customize the AI experience
• Same informal, friendly tone with appropriate emoji usage (✨)

Additional Updates

• Updated the blog post's frontmatter tags to include "GitHub Copilot"
• Positioned the new section logically after the main overview table but before the templates section

The changes are minimal and surgical - only adding new content while preserving all existing structure and maintaining the author's distinctive voice and style.

Fixes #67.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.