Docs: Add docs for the `LAYOUT_*` macro to layouts (#3772)

When moving my planck layout to my let's split, I ran into this issue
because I had copied my planck layout from the default layout which just
used the `{}` array form for the keymap layers. I checked the docs, but
this bit wasn't clear to me. I'm sure @ishtob or @drashna helped me on
discord, but this seemed to be the logical place to add a helpful hint.

docs/feature_layouts.md

@@ -53,6 +53,8 @@ but the `LAYOUT_<layout>` variable must be defined in `<folder>.h` as well.
 
 ## Tips for Making Layouts Keyboard-Agnostic
 
+### Includes
+
 Instead of using `#include "planck.h"`, you can use this line to include whatever `<keyboard>.h` (`<folder>.h` should not be included here) file that is being compiled:
 
     #include QMK_KEYBOARD_H
@@ -72,3 +74,7 @@ For example:
 ```
 
 Note that the names are lowercase and match the folder/file names for the keyboard/revision exactly.
+
+### Keymaps
+
+In order to support both split and non-split keyboards with the same layout, you need to use the keyboard agnostic `LAYOUT_<layout name>` macro in your keymap. For instance, in order for a Let's Split and Planck to share the same layout file, you need to use `LAYOUT_ortho_4x12` instead of `LAYOUT_planck_grid` or just `{}` for a C array.