feat(shields): Microdox V2 shield definition

* Refactor common parts of the Microdox sheild into a separate file. This is in preparation for adding Microdox V2 as another shield in the same directory.
* Refactor Microdox keymap into a common file in preparation for Microdox V2
* Add Microdox V2 shield definition
* Added a README to explain v1/v2 differences.

app/boards/shields/microdox/microdox_v2.zmk.yml

@@ -0,0 +1,13 @@
+file_format: "1"
+id: microdox_v2
+name: Microdox V2
+type: shield
+url: https://boardsource.xyz/store/5f2e7e4a2902de7151494f92
+requires: [pro_micro]
+exposes: [i2c_oled]
+features:
+  - keys
+  - display
+siblings:
+  - microdox_v2_left
+  - microdox_v2_right