Discuss layout macro

There is no mention or visualization of how to create a layout macro in `info.json` for split keyboards.

@fauxpark helped me tremendously in Discord by creating and providing these visualizations to aid my understanding:
https://cdn.discordapp.com/attachments/440868230475677698/1073409438712610907/image.png
https://cdn.discordapp.com/attachments/440868230475677698/1073409562373259364/image.png

I strongly feel these should be in the docs.

I think it best to include a short discussion in the [split keyboard page](https://docs.qmk.fm/#/feature_split_keyboard), explaining that row pins are automatically doubled when setting `SPLIT_KEYBOARD = yes` and that QMK views the physical layouts according to those pictures.

These are the pages I used to search for answers to the question:
https://docs.qmk.fm/#/feature_split_keyboard
https://docs.qmk.fm/#/porting_your_keyboard_to_qmk?id=layout-macros
https://docs.qmk.fm/#/hardware_keyboard_guidelines?id=ltkeyboard_namehgt

docs/feature_split_keyboard.md

@@ -79,6 +79,17 @@ If you're using a custom transport (communication method), then you will also ne
 SPLIT_TRANSPORT = custom
 ```
 
+### Layout Macro
+When setting `SPLIT_KEYBOARD = yes`, row pins are automatically doubled.
+This is because QMK views a split keyboard as follows, with the numbers
+representing row, column:
+
+IMG1
+
+IMG2
+
+Take care to define your layout in `info.json` accordingly.
+
 ### Setting Handedness
 
 By default, the firmware does not know which side is which; it needs some help to determine that. There are several ways to do this, listed in order of precedence.