Bring README and copilot-instructions up to date with proper configuration documentation

[Copilot]

This PR addresses the incomplete and outdated documentation in both README.md and copilot-instructions.md, with particular focus on the "Configuration Options" section that was cutting off mid-sentence.

Key improvements made:

📝 Fixed incomplete Configuration Options section

The README previously had an incomplete "Configuration Options" section that ended abruptly. Now it properly documents both available input parameters:

• input: Path to Dhall source directory (default: src)
• package-name: Package name for documentation (optional)

🔧 Made workflow truly reusable

Enhanced the GitHub Actions workflow to support both workflow_dispatch and workflow_call triggers with proper input definitions, allowing other repositories to actually use this as a reusable workflow.

📚 Updated examples and references

• Changed example usage from non-existent @v0.1 tag to @main reference
• Added comprehensive examples showing how to configure both input parameters
• Updated troubleshooting section with more specific guidance

🎯 Enhanced workflow functionality

The workflow now properly accepts and forwards both input and package-name parameters to the underlying dhall-docs action, making it fully configurable.

✅ Corrected copilot-instructions

Updated the copilot-instructions.md to accurately reflect:

• Current trigger configuration (both manual and reusable workflow)
• Available input parameters and their usage
• Proper usage patterns for the reusable workflow

The workflow is now properly documented and functional as a reusable GitHub Actions workflow that other repositories can easily integrate for Dhall documentation deployment.

Fixes #5.

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