[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