[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 <[email protected]>
Co-authored-by: aelovikov-intel <[email protected]>
Co-authored-by: Alexey Bader <[email protected]>
Co-authored-by: Greg Lueck <[email protected]>

Author: 5 people