docs: add docs to CB

Pretty much every class and method should have documentation now.

ref: N25B-295

src/control_backend/api/v1/endpoints/message.py

@@ -11,6 +11,14 @@ router = APIRouter()
 
 @router.post("/message", status_code=202)
 async def receive_message(message: Message, request: Request):
+    """
+    Generic endpoint to receive text messages.
+
+    Publishes the message to the internal 'message' topic via ZMQ.
+
+    :param message: The message payload.
+    :param request: The FastAPI request object (used to access app state).
+    """
     logger.info("Received message: %s", message.message)
 
     topic = b"message"