Upgrade to Zensical (MK Docs follow up) and other fixes - #50

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: DavidAnson/markdownlint-cli2-action@v20, tj-actions/changed-files@v40. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/