[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/readme.md

@@ -0,0 +1,19 @@
+# IBM Trackpoint demonstration
+
+This is just a simple demo to show how to integrate IBM Trackpoint in QMK.
+
+Wiring used in the demonstration:  
+![Wiring example](https://i.imgur.com/8ghG2U8.png)
+
+Some documentation:
+
+- [How to wire IBM Trackpoint](https://github.com/alonswartz/trackpoint)
+
+Keyboard Maintainer: [The QMK Community](https://github.com/qmk)  
+Hardware Supported: Teensy  
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/trackpoint:default
+
+See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).