Pin GitHub Actions to full-length commit SHAs - #124

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02, DavidAnson/markdownlint-cli2-action@eb5ca3ab411449c66620fe7f1b3c9e10547144b0, peaceiris/actions-mdbook@ee69d230fe19748b7abf22df32acaa93833fad08. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/