docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

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

docs/src/components/codes/Footnote.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 PropTypes from "prop-types";
+
+export default function Footnote({ children, symbol, id }) {
+  return (
+    <div className="footnote" id={id}>
+      <div className="symbol">{symbol}</div>
+      <div className="content">{children}</div>
+    </div>
+  );
+}
+
+Footnote.propTypes = {
+  children: PropTypes.element.isRequired,
+  symbol: PropTypes.oneOfType([PropTypes.string, PropTypes.number]).isRequired,
+  id: PropTypes.string.isRequired,
+};