docs: split design.md into per-layer docs (L1-L5) + a slim index - #157

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: amannn/action-semantic-pull-request@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/