[Keyboard] Add split_3x5_3 layout support to boardsource/microdox (#13323)

keyboards/boardsource/microdox/rules.mk

@@ -21,3 +21,5 @@ RGBLIGHT_ENABLE = yes       # Enable keyboard RGB underglow
 BLUETOOTH_ENABLE = no       # Enable Bluetooth
 AUDIO_ENABLE = no           # Audio output
 SPLIT_KEYBOARD = yes
+
+LAYOUTS = split_3x5_3