[SYCL][Doc] Add free function kernel extension spec (#11861)

Add a proposed extension spec that allows a kernel to be defined as a
plain function, where the function arguments are the kernel arguments.
This is useful for the sycl_ext_oneapi_kernel_compiler because it allows
a kernel to be defined in isolation from its host code.

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Co-authored-by: Tom Honermann <[email protected]>

Author: gmlueck