Add comprehensive GitHub Copilot instructions for static documentation site

[Copilot]

Created comprehensive .github/copilot-instructions.md file providing detailed guidance for GitHub Copilot coding agents working with this repository.

What this adds

The instructions file provides:

• Clear repository understanding: Explains this is a static GitHub Pages deployment repository containing pre-built React Styleguidist documentation, not a source code repository
• Exact commands with validation: All commands have been tested and work correctly, including:
  • Local site serving with python3 -m http.server 8000 (~2 seconds startup)
  • File integrity checks using test -f commands
  • Quick site testing with curl verification
• Comprehensive validation scenarios: Step-by-step instructions for testing all site functionality:
  • Component navigation and filtering
  • Isolated component views
  • Code example expansion/collapse
  • External link verification
  • Service worker functionality
• Critical warnings: Clear guidance that this is a deployment repository with no build processes, preventing agents from attempting to run npm/build commands that don't exist

Repository Context

This repository serves documentation for University of Cambridge UI Automation React components including:

• AppBar - Application header with responsive hamburger menu
• NavigationPanel - Side navigation with user avatar, sections, and footer
• ChooseColumnsDialog - Interactive column selection dialog
• 7 additional navigation-related components

The site is built with React Styleguidist and includes interactive examples, code previews, and component isolation features.

Validation Performed

All instructions have been thoroughly tested:

• ✅ Site serves correctly and loads all 10 components
• ✅ Interactive features work (filtering, navigation, isolated views)
• ✅ All bash commands execute successfully
• ✅ External links and React Styleguidist attribution verified
• ✅ Service worker registration confirmed

This ensures agents can immediately work effectively with the repository using copy-paste commands that work every time.

Fixes #3.

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