docs: add sequence diagrams and per-audience mental models - #454

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, alandefreitas/cpp-actions/create-changelog@v1.9.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/