docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

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

docs/src/components/codes/Context.jsx

@@ -0,0 +1,17 @@
+/*
+ * 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 Context({ children }) {
+  return <p className="context">{children}</p>;
+}
+
+Context.propTypes = {
+  children: PropTypes.oneOfType([PropTypes.element, PropTypes.string])
+    .isRequired,
+};