feat(endpoints): rename behavior to outputs

"Outputs" is probably easier for most people to understand than "endpoints".

app/dts/bindings/behaviors/zmk,behavior-outputs.yaml

@@ -0,0 +1,10 @@
+#
+# Copyright (c) 2020, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+#
+
+description: Output Selection Behavior
+
+compatible: "zmk,behavior-outputs"
+
+include: one_param.yaml