Meta: Split serial console guide into a separate section

Now that we boot to desktop on the Pi 4 and 5, connecting a serial cable
is not absolutely necessary anymore.

Documentation/RunningOnRaspberryPi.md

@@ -35,8 +35,6 @@ SERENITY_RUN=raspi3b Meta/serenity.sh gdb aarch64
 
 ## Running on real hardware using a USB drive
 
-### Step 0: Build a SerenityOS disk image for Raspberry Pis
-
 In order to create a disk image, you first need to build SerenityOS by running
 
 ```console
@@ -69,6 +67,11 @@ Write this image to the USB drive you want to use to boot Serenity using a comma
 sudo cp Build/aarch64/<disk image> /dev/<usb drive> && sync
 ```
 
+## Serial debug output
+
+Debug output is written to the serial console if `serial_debug` is included in the cmdline.
+If you followed the above instructions, this option should already be enabled.
+
 ### Step 1: Connect your Raspberry Pi to your PC using a UART cable
 
 Please follow one of the existing guides (for example [here](https://scribles.net/setting-up-serial-communication-between-raspberry-pi-and-pc)) and make sure UART is working on Raspberry Pi OS before proceeding.