docs(extensions/nanoarrow_device): Document device extension

[paleolimbot]

As I was reminded in #488 (comment) , we have very minimal top-level documentation for the nanoarrow device extension (we do have inline function documentation, though). After some of the larger-scale details are sorted out, the high-level documentation and/or a getting started article should be added (which will also help flush out the scope of the extension and how it might be used).

[paleolimbot]

The old README:

nanoarrow device option

The nanoarrow device options provides a similar set of tools as the core nanoarrow C API
extended to the
Arrow C Device
interfaces in the Arrow specification.

Currently, this option provides an implementation of CUDA devices
and an implementation for the default Apple Metal device on MacOS/M1.
These implementation are preliminary/experimental and are under active
development.

Example

struct ArrowDevice* gpu = ArrowDeviceMetalDefaultDevice();
// Alternatively, ArrowDeviceCuda(ARROW_DEVICE_CUDA, 0)
// or  ArrowDeviceCuda(ARROW_DEVICE_CUDA_HOST, 0)
struct ArrowDevice* cpu = ArrowDeviceCpu();
struct ArrowArray array;
struct ArrowDeviceArray device_array;
struct ArrowDeviceArrayView device_array_view;

// Build a CPU array
ASSERT_EQ(ArrowArrayInitFromType(&array, NANOARROW_TYPE_STRING), NANOARROW_OK);
ASSERT_EQ(ArrowArrayStartAppending(&array), NANOARROW_OK);
ASSERT_EQ(ArrowArrayAppendString(&array, ArrowCharView("abc")), NANOARROW_OK);
ASSERT_EQ(ArrowArrayAppendString(&array, ArrowCharView("defg")), NANOARROW_OK);
ASSERT_EQ(ArrowArrayAppendNull(&array, 1), NANOARROW_OK);
ASSERT_EQ(ArrowArrayFinishBuildingDefault(&array, nullptr), NANOARROW_OK);

// Convert to a DeviceArray, still on the CPU
ASSERT_EQ(ArrowDeviceArrayInit(cpu, &device_array, &array), NANOARROW_OK);

// Parse contents into a view that can be copied to another device
ArrowDeviceArrayViewInit(&device_array_view);
ArrowArrayViewInitFromType(&device_array_view.array_view, string_type);
ASSERT_EQ(ArrowDeviceArrayViewSetArray(&device_array_view, &device_array, nullptr),
          NANOARROW_OK);

// Copy to another device. For some devices, ArrowDeviceArrayMoveToDevice() is
// possible without an explicit copy (although this sometimes triggers an implicit
// copy by the driver).
struct ArrowDeviceArray device_array2;
device_array2.array.release = nullptr;
ASSERT_EQ(
    ArrowDeviceArrayViewCopy(&device_array, &device_array_view, gpu, &device_array2),
    NANOARROW_OK);