Add sycl_ext_oneapi_free_function_kernels to the supported extensions list

The [sycl_ext_oneapi_free_function_kernels](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/experimental/sycl_ext_oneapi_free_function_kernels.asciidoc) is currently experimental extension while actual products such as Pytorch start adding this extension to the versions going to be shipped. Can this extension be promoted from experimental to supported?

See: https://github.com/intel/torch-xpu-ops/blob/9022a83389905509033d02cda8005c21be9c976c/src/comm/SYCLHelpers.h#L289

CC: @gmlueck, @EikanWang, @guangyey

Start with the experimental extension document at sycl/doc/extensions/experimental/sycl_ext_oneapi_free_function_kernels.asciidoc, then locate the repository's supported extensions list and compare its promotion conventions. Use the referenced torch-xpu-ops SYCLHelpers.h usage as context; done means the extension is listed as supported rather than experimental, with any corresponding documentation checks passing.

Written by the indexing model from the issue text.