Moved an example of language_basics.rst into the examples directory for testing.

docs/examples/userguide/language_basics/override.pyx

+from __future__ import print_function
+
+cdef class A:
+    cdef foo(self):
+        print("A")
+
+cdef class B(A):
+    cpdef foo(self):
+        print("B")
+
+class C(B):  # NOTE: not cdef class
+    def foo(self):
+        print("C")

docs/src/userguide/language_basics.rst

@@ -510,21 +510,9 @@ Overriding in extension types
 
 When subclassing an extension type with a Python class,
 ``def`` methods can override ``cpdef`` methods but not ``cdef``
-methods::
+methods:
 
-    from __future__ import print_function
-
-    cdef class A:
-        cdef foo(self):
-            print("A")
-
-    cdef class B(A):
-        cpdef foo(self):
-            print("B")
-
-    class C(B):         # NOTE: not cdef class
-        def foo(self):
-            print("C")
+.. literalinclude:: ../../examples/userguide/language_basics/override.pyx
 
 If ``C`` above would be an extension type (``cdef class``),
 this would not work correctly.