docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

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

docs/src/components/codes/OsSupport.jsx

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: CC-BY-NC-SA-4.0
+ */
+
+import React from "react";
+import PropTypes from "prop-types";
+import OsSupportIcon from "./OsSupportIcon";
+import FootnoteRefs from "./FootnoteRefs";
+
+export default function OsSupport({ value, footnotes = [] }) {
+  return (
+    <>
+      <OsSupportIcon value={value} />
+      {footnotes.length > 0 ? (
+        <FootnoteRefs footnotes={footnotes} />
+      ) : undefined}
+    </>
+  );
+}
+
+OsSupport.propTypes = {
+  value: PropTypes.oneOf([true, false, null]),
+  footnotes: PropTypes.array.isRequired,
+};