Clarify that npm commands should be executed in the frontend folder

[pviotti]

Some people, in the second step (02-building-the-ui.md), have executed the command npm install react-bootstrap @types/react-bootstrap in the root folder of the project instead of the frontend folder. The missing dependencies have caused errors running npm test such as Cannot find module 'react' from 'Accordion.js' at Resolver.resolveModule.
We should clarify that all npm commands must be executed in the frontend folder.