[SYCL][ESIMD] Intel "Explicit SIMD" extension documentation draft. (#1731)

* [SYCL][SIMD] Intel "Explicit SIMD" extension documentation initial version.

Contributors (alphabetic order):
Konst Bobrovsky
Peter Caday
Gang Chen
Kai Yu Chen
Ken Lueh
Wei Pan
Xinmin Tian
+ other people who reviewed the spec.

Signed-off-by: Konstantin S Bobrovsky <[email protected]>
Co-authored-by: John Pennycook <[email protected]>
Co-authored-by: rolandschulz <[email protected]>

Author: 3 people