added links to op and type contracts

doc/advanced_tutorial/ex1/op.txt

@@ -7,6 +7,8 @@ Now that we have a ``double`` type, we have yet to use it to perform
 computations. We'll start by defining multiplication.
 
 
+.. _op_contract:
+
 Op's contract
 =============
 

doc/advanced_tutorial/ex1/type.txt

@@ -5,6 +5,8 @@ Making the double type
 ======================
 
 
+.. _type_contract:
+
 Type's contract
 ===============