[SYCL][DOC] Graph API extensions (#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 <[email protected]>
Co-authored-by: Ronan Keryell <[email protected]>
Co-authored-by: Ewan Crawford <[email protected]>
Co-authored-by: Ben Tracy <[email protected]>
Co-authored-by: Ewan Crawford <[email protected]>
Co-authored-by: Ben Tracy <[email protected]>
Co-authored-by: Greg Lueck <[email protected]>