Add PR change-validation workflow for wiki/docs/assets content

[nevalsar]

Summary

This PR adds a pull request validation workflow that checks changed content files in wiki/, docs/, and assets/.

What Changed

• Added workflow:
  • .github/workflows/pr-change-validation.yml
• Added CI validation scripts:
  • .github/scripts/validate_changed_files_ci.py
  • .github/scripts/validate_markdown_ci.py
  • .github/scripts/validate_images_ci.py
  • .github/scripts/validate_yaml_ci.py

Validation Behavior

• On PRs, validates only changed files (not whole repository).
• Scope is limited to:
  • wiki/
  • docs/
  • assets/
• Markdown checks include:
  • filename format checks (kebab-case with allowed exceptions),
  • broken internal link detection,
  • image reference checks.
• Parser guards avoid false positives in fenced code blocks and math spans.
• Supports manual full-scope run via workflow_dispatch.