fix & simplify dev setup guide - #688

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, actions/setup-node@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

'e' is defined but never used. Allowed unused caught errors must match /^_/u

'err' is defined but never used. Allowed unused caught errors must match /^_/u

'e' is defined but never used. Allowed unused caught errors must match /^_/u

Insert `⏎·······`

Delete `⏎`

Delete `⏎`