docs(selfhost): describe what the starters actually ship for critical secrets - #6365

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/