arrow

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 <vyasr@nvidia.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>