docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

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

docs/src/components/codes/OsLegend.jsx

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: CC-BY-NC-SA-4.0
+ */
+
+import React from "react";
+import operatingSystems from "@site/src/data/operating-systems";
+
+export default function OsLegend() {
+  return (
+    <div className="codes os legend">
+      {operatingSystems.map(({ key, className, heading, title }) => (
+        <div key={key} className={"os " + className}>
+          <span className="heading">{heading}</span>
+          <span className="title">{title}</span>
+        </div>
+      ))}
+    </div>
+  );
+}
+
+OsLegend.propTypes = {};