[SYCL][Doc] Cluster Group Extension Document (intel#13594)

Initial public working draft for thread block cluster support in SYCL,
intended to get feedback.

Contains the proposal for - 
1. Launching a kernel with cluster group
2. Accessing the various `ids` associated with the cluster_group from
the kernel
3. Cluster level barrier
4. Accessing another workgroup's local memory

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Co-authored-by: Greg Lueck <gregory.m.lueck@intel.com>
Co-authored-by: Gordon Brown <gordon@codeplay.com>
Co-authored-by: John Pennycook <john.pennycook@intel.com>
Co-authored-by: Ruyman <ruyman@codeplay.com>