fix: Rationalize custom icons to avoid duplication (#1639)

docs/usage/README.md

@@ -11,6 +11,10 @@ See [2-way audio](2-way-audio.md) for documentation on using 2-way audio.
 
 See [Casting](casting.md) for documentation on casting the card.
 
+### Custom Icons
+
+See [Custom Icons](custom-icons.md) for custom icons this card supports.
+
 ### URL Actions
 
 See [URL Actions](url-actions.md) for documentation on acting based on URL contents.