[SPIR-V][Doc] add draft SPIR-V extension for 16-bit atomics (#20009)
Adds the proposed SPV_INTEL_shader_atomic_bfloat16 extension
specification, which extends the floating-point atomic support in SPIR-V
to support floating-point atomic addition, minimum, and maximum on
16-bit bfloat16 floating-point numbers in memory.
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Signed-off-by: Ben Ashbaugh <ben.ashbaugh@intel.com>