Added missing-class-docstring message example (#6250)

Author: matusvalo

doc/data/messages/m/missing-class-docstring/bad.py

@@ -0,0 +1,5 @@
+class Person:  # [missing-class-docstring]
+
+    def __init__(self, first_name, last_name):
+        self.first_name = first_name
+        self.last_name = last_name

doc/data/messages/m/missing-class-docstring/good.py

@@ -0,0 +1,6 @@
+class Person:
+    """Class representing a person"""
+
+    def __init__(self, first_name, last_name):
+        self.first_name = first_name
+        self.last_name = last_name