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

keyboards/handwired/atreus50/readme.md

@@ -1,7 +1,9 @@
-Handwired Atreus50
-==================
+# Handwired Atreus50
 
-This firmware is for a Handwired Atreus50 using an Arduino Pro Micro.
+This firmware is for a handwired Atreus50 using an Arduino Pro Micro.
+
+Keyboard Maintainer: [The QMK Community](https://github.com/qmk)  
+Hardware Supported: Arduino Pro Micro  
 
 ## Pinout
 
@@ -11,6 +13,12 @@ The following pins are used:
 
 ## Compiling and loading the firmware
 
-To build the firmware, run `make`.
+Make example for this keyboard (after setting up your build environment):
 
-To flash the firemware onto the microcontroller, run `make avrdude`, and press the reset button.
+    make handwired/atreus50:default
+
+To flash the firmware onto the microcontroller, run `make avrdude`, and press the reset button.
+
+    make handwired/atreus50:default:avrdude
+
+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).