[SYCL][Doc] Change graph_support_level namespace

Address [Steffen's
feedback](intel#5626 (comment))
and move the `info::device::graph_support_level` to enum up a namespace
level to `info::graph_support_level`.

Using `info::device::graph_support` to return a `info::graph_support_level` is
analogous with the `info::device::device_type` query in the main
spec which returns a `info::device_type`. Or `info::device::local_mem_type`
which returns a `info::local_mem_type`.

Author: EwanC

sycl/doc/extensions/proposed/sycl_ext_oneapi_graph.asciidoc

@@ -330,15 +330,16 @@ class depends_on {
 // Device query for level of support
 namespace info {
 namespace device {
+
 struct graphs_support;
 
+} // namespace device
+
 enum class graph_support_level {
   unsupported,
   native,
   emulated
 };
-
-} // namespace device
 } // namespace info
 
 class node {};