what to do if mbed_app.json is not in the application root

# Docs issue template

## Page link
https://os.mbed.com/docs/mbed-os/v6.16/bare-metal/using-the-bare-metal-profile.html

## Expected to find...
"Open mbed_app.json (in the root of the application) and replace it with the following content:"

## Found
Did not find mbed_app.json in the application root. So, what to do in that case would be good in the docs.
