[SYCL][CODEOWNERS] Update CODEOWNERS for SYCL-Graph specification

After the merge of the [SYCL-Graph extension spec PR](intel#5626)
add @intel/sycl-graphs-reviewers as reviewers of the specification
[document](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_graph.asciidoc).

Used `**` for the extension status directory as the extension
is expected to shortly move from proposed to experimental.

Author: EwanC

.github/CODEOWNERS

@@ -120,6 +120,7 @@ sycl/include/sycl/ext/oneapi/experimental/graph.hpp @intel/sycl-graphs-reviewers
 sycl/source/detail/graph_impl.cpp @intel/sycl-graphs-reviewers
 sycl/source/detail/graph_impl.hpp @intel/sycl-graphs-reviewers
 sycl/unittests/Extensions/CommandGraph.cpp @intel/sycl-graphs-reviewers
+sycl/doc/extensions/**/sycl_ext_oneapi_graph.asciidoc @intel/sycl-graphs-reviewers
 
 # syclcompat library
 sycl/**/syclcompat/ @intel/syclcompat-lib-reviewers