handwired/terminus_mini: refactor, Configurator support, and readme cleanup (#5292)

handwired/terminus_mini: refactor, Configurator support, and readme cleanup (#5292)

* handwired/terminus_mini: refactor

- white space changes in terminus_mini.h (alignment)
- keymap now uses #include QMK_KEYBOARD_H
- refactored TMK macros into QMK process_record_user
- refactored TMK layer functions into QMK keycodes
- removed redundant keycode definitions

* handwired/terminus_mini: Configurator support

* handwired/terminus_mini: readme cleanup

- completed keyboard description
- removed Hardware Availability line (it was blank)
- updated Docs links