docs(codes): Add (key) codes documentation

Create codes documentation for standardized keys.

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

docs/src/footnotes.js

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: CC-BY-NC-SA-4.0
+ */
+
+import footnotes from "./data/footnotes";
+
+export function getFootnote(id) {
+  const footnote = footnotes[id];
+  if (typeof footnote != "undefined") {
+    return footnote;
+  }
+}