Could we use `npm install -D ts-jest` instead of `npm add ts-jest`?

[ziyang-works]

Questions regarding https://cap.cloud.sap/docs/node.js/typescript#testing-with-ts-jest

1. npm add is an alias for much more common npm install (source). Could we use the latter to avoid confusion?
2. ts-jest is a dev dependency, but npm add adds it to dependencies. We should use npm install -D ts-jest instead

[renejeglinsky]

Thank you for opening this issue and the corresponding PR.

All the best,
René