pybricks.tools: Document hub_menu.

Fixes https://github.com/pybricks/pybricks-api/issues/144

examples/pup/tools/hub_menu.py

@@ -0,0 +1,16 @@
+from pybricks.tools import hub_menu
+
+# This example assumes that you have three other programs in Pybricks Code,
+# called "fly_mission", "drive_mission", and "zigzag". This example creates a
+# menu that lets you pick which one to run.
+
+# Choose a letter.
+selected = hub_menu("F", "D", "Z")
+
+# Based on the selection, run a program.
+if selected == "F":
+    import fly_mission
+elif selected == "D":
+    import drive_mission
+elif selected == "Z":
+    import zigzag