Add documentation on composite devices to user and beginner guides

[ndgrigorian]

Both user's guide and beginner's guide have sections on devices.

Composite devices should be covered here.

Can look to guides like https://www.intel.com/content/www/us/en/developer/articles/technical/flattening-gpu-tile-hierarchy.html https://www.intel.com/content/www/us/en/docs/oneapi/optimization-guide-gpu/2025-0/multi-gpu-and-multi-stack-architecture-and.html

and the compiler itself https://github.com/intel/llvm/blob/71ca51fb52ccf0bd8f03d33cf9ea2962cf254a7f/sycl/doc/MultiTileCardWithLevelZero.md https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/experimental/sycl_ext_oneapi_composite_device.asciidoc