docs: add Prompts feature page and linked agent instructions - #898

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, actions/setup-node@v4, actions/upload-artifact@v4, dawidd6/action-download-artifact@v6, peter-evans/create-or-update-comment@v4, peter-evans/find-comment@v3, pnpm/action-setup@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/