✨ Set up Copilot instructions and comprehensive documentation

[Copilot]

This PR implements Copilot instructions for the repository according to GitHub's best practices for coding agents. The changes provide comprehensive context and guidance to help GitHub Copilot understand the purpose and structure of this reusable GitHub Actions workflow.

What was added:

.github/copilot-instructions.md - A comprehensive guide that includes:

• Repository overview explaining this is a reusable GitHub Actions workflow for Dhall documentation deployment
• Detailed breakdown of the workflow functionality and components
• Dhall-specific context and domain knowledge
• Modification guidelines for developers working with the workflow
• Testing approaches and common troubleshooting scenarios
• Clear documentation of dependencies and requirements

README.md - Complete repository documentation featuring:

• Clear description of the workflow's purpose and capabilities
• Step-by-step usage instructions for copying the workflow to other repositories
• Feature highlights including manual triggers, concurrency control, and proper permissions
• Requirements and customization options
• Reference to Dhall language resources

These files ensure that GitHub Copilot has the necessary context to:

• Understand this is a specialized workflow for Dhall documentation
• Provide relevant suggestions for GitHub Actions modifications
• Offer appropriate guidance for workflow customization
• Recognize the domain-specific requirements of Dhall projects

The instructions follow GitHub's recommended structure for coding agent guidance while being tailored specifically to this repository's unique purpose as a reusable workflow template.

Fixes #3.

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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.