[Keyboard] XD75 refactor (#5420)

* Refactor layout macro names

Update the names of the layout macros to reflect QMK's desired usage.

- KEYMAP renamed to LAYOUT
  - colinta keymap updated accordingly
- KC_KEYMAP renamed to LAYOUT_kc
  - mtdjr keymap updated accordingly

* Update xd75.h to use #pragma once include guard

* Update keymaps to use QMK_KEYBOARD_H

* Refactor default keymap

- updated to use layout macro
- remove deprecated action_get_macro function
- add functions from QMK keymap.c template
  - process_record_user
  - matrix_init_user
  - matrix_scan_user
  - led_set_user

* Update supporting files

- updated config.h files to use #pragma once include guard
- remove errant QUANTUM_DIR code block

* Rename readme files to fully lowercase filenames

* Restructure keyboard readme

Updated the readme to better align with the current QMK template.

* Bugfixes to supporting files

keyboards/xd75/keymaps/mtdjr/config.h

@@ -14,10 +14,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef CONFIG_USER_H
-#define CONFIG_USER_H
-
-#include "../../config.h"
+#pragma once
 
 // place overrides here
 #undef MANUFACTURER
@@ -25,5 +22,3 @@
 #define MANUFACTURER    mtdjr
 #define PRODUCT         XD75
 #define DESCRIPTION     XD75Re
-
-#endif