Melody96 Configurator updates and minor refactoring (#6365)

* Add Configurator layout data for LAYOUT_hotswap

* Add LAYOUT_std60_split_num0

Requested by 李小安#9728 on QMK Discord.

Standard 60% ANSI layout for the alphanumeric region, with a split-0 Numpad.

Includes a sample keymap.

* Update Docs links on readme

* Change melody96.h to use #pragma once include guard

* Change config.h to use #pragma once include guard

* Add readme for default_std60_split_num0 keymap

keyboards/melody96/config.h

@@ -1,5 +1,4 @@
-#ifndef CONFIG_H
-#define CONFIG_H
+#pragma once
 
 #include "config_common.h"
 
@@ -46,5 +45,3 @@
 #define RGBLIGHT_SAT_STEP 8
 #define RGBLIGHT_VAL_STEP 8
 #endif
-
-#endif