Document approaches for site builds on top of EESSI - #778

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@a5ac7e51b41094c92402da3b24376905380afc29, actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/