Renamed the notion of handler classes to plugin classes in the Studio Python API

This change refactors the Python API to use the term "plugins" instead of "handlers" for classes containing decorated handler methods. The term handler is still used for an annotated method of a plugin class that handles a specific message type. Also improved some documentation.

PiperOrigin-RevId: 962150099
Change-Id: I34a8cc410cd784b088605490cfa3baccea5c9e71

python/mujoco/experimental/studio/sample/implot.py

@@ -238,7 +238,7 @@ def main(argv: list[str]) -> None:
 
   with launch_passive.launch_passive(
       config,
-      viewer_handlers=[viewer_app.ViewerApp(), BodyInspector()],
+      viewer_plugins=[viewer_app.ViewerApp(), BodyInspector()],
   ) as handle:
     handle.send_to_viewer(messages.ModelEvent(model=model))