boards: nrf_bsim: Document how to use the console with bsim

This commit describes how to build and run applications using
the console. This documentation is added because it may
not be obvious how this is done. That is, sometimes a user
may believe that the terminal launching the application is the
the console which is not the case.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>