docs: rewrite README and docs/ for accuracy and voice; split README into guides - #458

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, gitleaks/gitleaks-action@ff98106e4c7b2bc287b24eaf42907196329070c7. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/