docs: reorganize docs/ into design_docs/, developer_docs/, and user-guide/ - #265

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, mozilla-actions/sccache-action@v0.0.4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/