[SYCL][Doc] Extended group load/store APIs proposal (#7593)

An initial draft of extended group load/store APIs to provide
capabilities to work with temporary memory buffers and load/store
multiple elements per work item.

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Co-authored-by: John Pennycook <john.pennycook@intel.com>
Co-authored-by: aelovikov-intel <andrei.elovikov@intel.com>
Co-authored-by: Alexey Bader <alexey.bader@intel.com>
Co-authored-by: Greg Lueck <gregory.m.lueck@intel.com>