docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

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

docs/src/components/codes/Footnotes.jsx

@@ -0,0 +1,30 @@
+/*
+ * 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 Footnote from "./Footnote";
+
+export default function Footnotes({ footnotes = [], id }) {
+  return (
+    <div className="footnotes">
+      <a id={id} className="anchor" />
+      <div className="label">Notes</div>
+      <div className="notes">
+        {footnotes.map((footnote) => (
+          <Footnote key={footnote.id} {...footnote}>
+            {footnote.value}
+          </Footnote>
+        ))}
+      </div>
+    </div>
+  );
+}
+
+Footnotes.propTypes = {
+  footnotes: PropTypes.array.isRequired,
+  id: PropTypes.string.isRequired,
+};