Add base Astro + Nebari Starlight docs site (experimental) - #5

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: thollander/actions-comment-pull-request@24bffb9b452ba05a4f3f77933840a6a841d1b32b. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/