doc: application development: brief about setting QEMU gdbserver listener

Briefly explain how a user can change their gdbserver listening device via
the Kconfig option or via shell environment. User is reminded about caveats
regarding using environment variables with CMake systems. Finally, another
invokation of QEMU-with-gdb via west from root directory is presented.

Signed-off-by: Alp Sayin <alpsayin@gmail.com>