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.dtsi

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "microdox_common.dtsi"
+
+/ {
+    kscan0: kscan {
+        compatible = "zmk,kscan-gpio-matrix";
+        label = "KSCAN";
+        diode-direction = "col2row";
+    };
+};