Moved the second code snippet from cdef_classes.rst to the examples directory...

Moved the second code snippet from cdef_classes.rst to the examples directory to add it to the tests.

docs/examples/tutorial/cdef_classes/math_function_2.pyx

+cdef class Function:
+    cpdef double evaluate(self, double x) except *:
+        return 0

docs/src/tutorial/cdef_classes.rst

@@ -33,11 +33,9 @@ Cython code and pure Python code.
 So far our integration example has not been very useful as it only
 integrates a single hard-coded function. In order to remedy this,
 with hardly sacrificing speed, we will use a cdef class to represent a
-function on floating point numbers::
+function on floating point numbers:
 
-  cdef class Function:
-      cpdef double evaluate(self, double x) except *:
-          return 0
+.. literalinclude:: ../../examples/tutorial/cdef_classes/math_function_2.pyx
 
 The directive cpdef makes two versions of the method available; one
 fast for use from Cython and one slower for use from Python. Then::