Keyboard: handwired/atreus50 Refactor, Configurator support, and readme cleanup ...

Keyboard: handwired/atreus50 Refactor, Configurator support, and readme cleanup (#4530)

* handwired/atreus50: refactor

- layout macro renames:
  - KEYMAP is now LAYOUT
  - COMPACT_KEYMAP is now LAYOUT_kc
- keymap updates:
  - both keymaps now use #include QMK_KEYBOARD_H
  - removed redundant KC_TRNS and KC_NO definitions
  - default keymap now uses LAYOUT macro instead of LAYOUT_kc

* handwired/atreus50: readme cleanup

Reformatted the readme to be more closely aligned to current QMK
template, and fixed some typos/grammar.

* handwired/atreus50: Configurator support