Document running the app while the backend/frontend are running externally

[joaquimrocha]

The instructions for running Headlamp in development mode indicate that we do a couple of make commands. This will run the backend + frontend, allowing developers to test Headlamp in the browser. However, this doesn't test the app module, and if developers want to run the app, then it will trigger the backend again.

We do have an npm command for running only the app module which will use the already running backend/frontend: npm run dev-only-app

This issue is about adding the documentation for running the app in development mode without triggering the backend (the command above).

Notes

• See the file docs/development/_index.md at https://github.com/headlamp-k8s/headlamp/blob/main/docs/development/_index.md#build-the-app for where to add the documentation
• The command is to run the app when headlamp is being run in development mode with make run-backend and make run-frontend. The benefit of it is you can develop the frontend version of headlamp at the same time as the app version (without having to restart any commands).