[Keyboard] handwired/trackpoint: refactor and readme cleanup (#5325)

* handwired/trackpoint: refactor

- layout macro KEYMAP renamed to LAYOUT
- keymap now uses #include QMK_KEYBOARD_H

* handwired/trackpoint: readme cleanup

- fix wiring layout image URL
- update readme structure

* handwired/trackpoint: rename readme to lowercase

* handwired/trackpoint: update header files to use #pragma once

keyboards/handwired/trackpoint/trackpoint.h

@@ -1,13 +1,10 @@
-#ifndef TRACKPOINT_H
-#define TRACKPOINT_H
+#pragma once
 
 #include "quantum.h"
 
-#define KEYMAP( \
-	k00, k01, k02  \
+#define LAYOUT( \
+  k00, k01, k02  \
 ) \
 { \
-	{ k00, k01, k02} \
+  { k00, k01, k02} \
 }
-
-#endif