Sort the entries in the API specification table of contents (#365)

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Author: asmeurer

spec/API_specification/index.rst

@@ -7,20 +7,20 @@ API specification
    :caption: API specification
    :maxdepth: 3
 
-   function_and_method_signatures
    array_object
-   indexing
-   data_types
-   data_type_functions
-   type_promotion
    broadcasting
+   constants
    creation_functions
-   manipulation_functions
+   data_type_functions
+   data_types
    elementwise_functions
-   statistical_functions
+   function_and_method_signatures
+   indexing
    linear_algebra_functions
+   manipulation_functions
    searching_functions
-   sorting_functions
    set_functions
+   sorting_functions
+   statistical_functions
+   type_promotion
    utility_functions
-   constants