fix: remove duplicated markdown and themes config keys (main is red) - #36

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, DavidAnson/markdownlint-cli2-action@v18.0.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/