sycl : Improve SYCL doc (#23025)

sycl : Improve SYCL doc (#23025)

* Tidy up SYCL doc a bit

- Add explicit links to referenced items
- Fix spelling errors

Signed-off-by: Todd Malsbary <redacted>
* Correct documented default for GGML_SYCL_GRAPH

The default is ON, not OFF:

  $ cmake -LAH -B build | grep GGML_SYCL_GRAPH
  ...
  GGML_SYCL_GRAPH:BOOL=ON

Signed-off-by: Todd Malsbary <redacted>
* Move docker instructions from SYCL.md to docker.md

This makes them directly accesible from the Quick Start section
of the top-level README.md.

Signed-off-by: Todd Malsbary <redacted>
* Refer to intel.Dockerfile for ARGs and their defaults

The defaults are always changing; this avoids accuracy errors
from duplicating the information.

Signed-off-by: Todd Malsbary <redacted>
* Remove mention of Nvidia in SYCL row of backend table

This support was removed in 2026.02 - refer to the SYCL.md News.

Signed-off-by: Todd Malsbary <redacted>
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Signed-off-by: Todd Malsbary <redacted>