docs: Added ADR template - #73

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: slackapi/slack-github-action@91efab103c0de0a537f72a35f6b8cda0ee76bf0a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/