Generate comprehensive README for Dhall documentation deployment workflow

[Copilot]

This PR adds a comprehensive README.md file to document the GitHub Actions workflow template for deploying Dhall documentation to GitHub Pages.

The README includes:

• Clear overview explaining the workflow's purpose and functionality
• Step-by-step setup guide with prerequisites and GitHub Pages configuration
• Complete workflow example that users can copy and customize
• Configuration options for different source directories and trigger events
• Repository structure recommendations for optimal organization
• Troubleshooting section covering common issues and solutions
• Permissions documentation explaining required GitHub Actions permissions

The documentation transforms this repository from having an empty README to providing comprehensive guidance for users who want to implement automated Dhall documentation deployment in their projects.

Example workflow usage:

- name: Generate docs
  uses: nikita-volkov/dhall-docs.github-action@v0.3
  with:
    input: src  # or your custom Dhall source directory

Fixes #1.

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[Copilot]

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