Add documentation link validation workflow

[Nokhrin]

This PR adds automated documentation link validation to the progit2 repository using the docs-validator tool.

What was done

• Integrated documentation link validation into the CI/CD pipeline
• Configured automated scanning of all AsciiDoc documentation files
• Generated comprehensive validation reports in Markdown format
• Uploaded validation reports as artifacts for review

Validation Results

https://github.com/Nokhrin/progit2/actions/runs/27497642111/job/81275058615

The initial validation scan identified:

• 50 errors - Broken external links that need attention
• 142 warnings - Files without incoming links and links requiring manual verification
• 113 files scanned across the entire documentation
• 260 external links checked (83 broken)
• 6 internal links checked (all valid)

Benefits

1. Improved documentation quality: Catches broken links before they reach readers
2. Automated quality control: Runs on every push and pull request
3. Actionable reports: Detailed Markdown reports uploaded as artifacts
4. Early detection: Identifies link rot and structural issues proactively
5. Maintainability: Helps maintain documentation integrity over time

Key findings

Most errors are in external GitHub repository links that no longer exist (e.g., old grit repositories, example projects). The validation also identified many files that are not linked from other documentation pages, which may need better navigation structure.

The full validation report is available in the workflow artifacts for detailed review.

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• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.