feat: AI Search plugin with indexing, search modal, and demo site - #633

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: lunariajs/action@a0594f1c5b8d55fb367d8df607d5e2541c99e77d. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/