Replace instances of KEYMAP with LAYOUT

Many instances in the QMK Docs referenced KEYMAP macros, which is outdated terminology.

Replaced most instances of KEYMAP with LAYOUT, to reflect the desired usage.

docs/feature_advanced_keycodes.md

@@ -11,7 +11,7 @@ People often define custom names using `#define`. For example:
 #define ALT_TAB LALT(KC_TAB)
 ```
 
-This will allow you to use `FN_CAPS` and `ALT_TAB` in your `KEYMAP()`, keeping it more readable.
+This will allow you to use `FN_CAPS` and `ALT_TAB` in your keymap, keeping it more readable.
 
 ## Caveats