docs: add docs to CB

Pretty much every class and method should have documentation now.

ref: N25B-295

src/control_backend/schemas/message.py

@@ -2,4 +2,8 @@ from pydantic import BaseModel
 
 
 class Message(BaseModel):
+    """
+    A simple generic message wrapper, typically used for simple API responses.
+    """
+
     message: str