[SYCL][Doc] Add kernel fusion extension proposal (#7098)

Add specification for the "sycl_ext_codeplay_kernel_fusion" extension
proposal, which allows user-driven kernel fusion of two or more kernels
in a single kernel launch.

Signed-off-by: Victor Perez <victor.perez@codeplay.com>
Co-authored-by: Lukas Sommer <lukas.sommer@codeplay.com>