[.github] Set up Copilot instructions

[Copilot]

Notes for Reviewers

This PR fixes #68

Adds .github/copilot-instructions.md to configure GitHub Copilot coding agent for this repository, per best practices.

Instructions tailored for Hugo/Docsy documentation site include:

• Project overview and technology stack (Hugo extended, Docsy, Go templates)
• Development workflow (make setup, make site, make build)
• Content guidelines for Markdown, Hugo shortcodes, and image handling
• Template and styling conventions
• WCAG 2.1 accessibility requirements
• Git workflow (Conventional Commits, DCO sign-off, branch naming)
• Troubleshooting common build/content issues
• Contribution checklist

Based on existing instructions from layer5io/layer5, adapted for this repository's Hugo-based stack.

Signed commits

• Yes, I signed my commits.

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

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Contributor Guide and Resources

• 📚 Instructions for contributing to documentation
  • Layer5 documentation site and source
• 🎨 Wireframes and designs for Layer5 site in Figma (open invite)
• 🙋🏾🙋🏼 Questions: Layer5 Discussion Forum and Layer5 Community Slack
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #861

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

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