docs: rewrite README around a Linux-first setup path - #176

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, DeterminateSystems/magic-nix-cache-action@v8. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Failed to save: <h2>Our services aren't available right now</h2><p>We're working to restore all services as soon as possible. Please check back soon.</p>0WPNmagAAAABj7xsLvZ2vTp9ydKNMWWpAUEhMMzBFREdFMDEwNwBFZGdl