GH-47243: [C++] Initialize arrow::compute in execution_plan_documentation_examples (#47227)

### Rationale for this change
fix example running error
```

        ****** Scan Example ******

Error occurred: No function registered with name: make_struct
```

### What changes are included in this PR?
call arrow::compute::initialize ahead

### Are these changes tested?
yes

### Are there any user-facing changes?
no

* GitHub Issue: #47243

Lead-authored-by: egolearner <[email protected]>
Co-authored-by: egolearner <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

Author: egolearner

cpp/examples/arrow/dataset_documentation_example.cc

@@ -19,6 +19,7 @@
 // intended to be paired with the documentation.
 
 #include <arrow/api.h>
+#include <arrow/compute/api.h>
 #include <arrow/compute/cast.h>
 #include <arrow/dataset/dataset.h>
 #include <arrow/dataset/discovery.h>
@@ -326,6 +327,8 @@ arrow::Result<std::shared_ptr<arrow::Table>> FilterPartitionedDataset(
 
 arrow::Status RunDatasetDocumentation(const std::string& format_name,
                                       const std::string& uri, const std::string& mode) {
+  ARROW_RETURN_NOT_OK(arrow::compute::Initialize());
+
   std::string base_path;
   std::shared_ptr<ds::FileFormat> format;
   std::string root_path;

cpp/examples/arrow/execution_plan_documentation_examples.cc

@@ -825,14 +825,13 @@ enum ExampleMode {
 };
 
 int main(int argc, char** argv) {
-  if (argc < 3) {
-    // Fake success for CI purposes.
-    return EXIT_SUCCESS;
+  int mode = argc > 1 ? std::atoi(argv[2]) : SOURCE_SINK;
+  std::string base_save_path = argc > 2 ? argv[2] : "";
+  arrow::Status status = arrow::compute::Initialize();
+  if (!status.ok()) {
+    std::cout << "Error occurred: " << status.message() << std::endl;
+    return EXIT_FAILURE;
   }
-
-  std::string base_save_path = argv[1];
-  int mode = std::atoi(argv[2]);
-  arrow::Status status;
   // ensure arrow::dataset node factories are in the registry
   arrow::dataset::internal::Initialize();
   switch (mode) {