Complete documentation overhaul.

docs/usage/README.md

@@ -0,0 +1,16 @@
+# Usage
+
+The usage of the card is intended to be reasonably self-explanatory. Some more
+complex situations / requirements are discussed here.
+
+### 2-way audio
+
+See [2-way audio](2-way-audio.md) for documentation on using 2-way audio.
+
+### Casting
+
+See [Casting](casting.md) for documentation on casting the card.
+
+### URL Actions
+
+See [URL Actions](url-actions.md) for documentation on acting based on URL contents.