[SYCL][DOC] Graph API extensions (intel#5626)

This extension is adding an API that targets separating definition and
execution of a SYCL Kernel Graph. This patch contains specification
only.

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Co-authored-by: John Pennycook <john.pennycook@intel.com>
Co-authored-by: Ronan Keryell <ronan@keryell.fr>
Co-authored-by: Ewan Crawford <ewan.cr@gmail.com>
Co-authored-by: Ben Tracy <benatracy@gmail.com>
Co-authored-by: Ewan Crawford <ewan@codeplay.com>
Co-authored-by: Ben Tracy <ben.tracy@codeplay.com>
Co-authored-by: Greg Lueck <gregory.m.lueck@intel.com>

Author: 7 people