Add documentation examples for overridden-final-method checker

doc/data/messages/o/overridden-final-method/bad.py

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+from typing import final
+
+
+class Base:
+    @final
+    def my_method(self):
+        pass
+
+
+class Subclass(Base):
+    def my_method(self):  # [overridden-final-method]
+        pass

doc/data/messages/o/overridden-final-method/details.rst

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+The message can't be emitted when using Python < 3.8.

doc/data/messages/o/overridden-final-method/good.py

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+from typing import final
+
+
+class Base:
+    @final
+    def my_method(self):
+        pass
+
+
+class Subclass(Base):
+    def my_other_method(self):
+        pass

doc/data/messages/o/overridden-final-method/related.rst

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+- `PEP 591 <https://peps.python.org/pep-0591/>`_