docs: add a migration guide from 1.x to 2.0 - #107

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: pnpm/action-setup@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/