[SYCL][Graph][Doc] Tidy of graph extension design doc (#13065)

Cleans up the design doc a bit with the following changes:

* Reflow lines to 80 chars.
* Fix some typos and change usages of UK English to US.
* Update intro that talks about emulation mode and only supporting L0.
* Add section on resources at the beginning where we can link public
information on the extension we've created.

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Co-authored-by: Ben Tracy <[email protected]>

Author: EwanC