GH-40318: [C++][Docs] Add documentation of array factories (#40373)

### Rationale for this change

These factory functions are generally useful and available, so documenting them helps external users find them without having to search the source code.

### What changes are included in this PR?

This PR adds the array factories in arrow/array/util.h into a doxygen group for array factories and adds that group to the Sphinx C++ API documentation.

### Are these changes tested?

I built the docs locally to verify.

### Are there any user-facing changes?

Nothing to the API, only docs.

* GitHub Issue: #40318

Authored-by: Vyas Ramasubramani <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

cpp/src/arrow/array/util.h

@@ -31,6 +31,10 @@
 
 namespace arrow {
 
+/// \defgroup array-factories Array factory functions
+///
+/// @{
+
 /// \brief Create a strongly-typed Array instance from generic ArrayData
 /// \param[in] data the array contents
 /// \return the resulting Array instance
@@ -65,6 +69,8 @@ ARROW_EXPORT
 Result<std::shared_ptr<Array>> MakeEmptyArray(std::shared_ptr<DataType> type,
                                               MemoryPool* pool = default_memory_pool());
 
+/// @}
+
 namespace internal {
 
 /// \brief Swap endian of each element in a generic ArrayData

docs/source/cpp/api/array.rst

@@ -27,6 +27,12 @@ Arrays
    :project: arrow_cpp
    :members:
 
+Factory functions
+=================
+
+.. doxygengroup:: array-factories
+   :content-only:
+
 Concrete array subclasses
 =========================
 
@@ -86,4 +92,4 @@ Utilities
 .. doxygenclass:: arrow::ArrayVisitor
    :project: arrow_cpp
    :members:
-   :undoc-members:
+   :undoc-members: