promise-types/groups/README.md: Formatted file and added syntax highlighting - #152

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, gabrielfalcao/pyenv-action@32ef4d2c861170ce17ded56d10329d83f4c8f797. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/