docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

Closes #218.  Fixes #308.  Ref #21.

docs/src/groups.js

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: CC-BY-NC-SA-4.0
+ */
+
+import groups from "./data/groups.js";
+
+export function getGroup(id) {
+  return groups[id] ?? null;
+}