Add documentation examples for dangerous-default-value. (#6881)

The `good.py` example is taken directly from the Python documentation.

Co-authored-by: Pierre Sassoulas <[email protected]>

Author: mbyrnepr2

doc/data/messages/d/dangerous-default-value/bad.py

@@ -0,0 +1,3 @@
+def whats_on_the_telly(penguin=[]):  # [dangerous-default-value]
+    penguin.append("property of the zoo")
+    return penguin

doc/data/messages/d/dangerous-default-value/details.rst

@@ -1 +1,7 @@
-You can help us make the doc better `by contributing <https://github.com/PyCQA/pylint/issues/5953>`_ !
+With a mutable default value, with each call the default value is modified, i.e.:
+
+.. code-block:: python
+
+    whats_on_the_telly() # ["property of the zoo"]
+    whats_on_the_telly() # ["property of the zoo", "property of the zoo"]
+    whats_on_the_telly() # ["property of the zoo", "property of the zoo", "property of the zoo"]

doc/data/messages/d/dangerous-default-value/good.py

@@ -1 +1,5 @@
-# This is a placeholder for correct code for this message.
+def whats_on_the_telly(penguin=None):
+    if penguin is None:
+        penguin = []
+    penguin.append("property of the zoo")
+    return penguin