Consider grouping extension names more strongly together

[EwanC]

From feedback:

• [SYCL][DOC] Graph API extensions intel/llvm#5626 (comment)
• [SYCL][DOC] Graph API extensions intel/llvm#5626 (comment)

Should command_graph simply be graph, and should we have a namespace for other objects (node, graph_state, depends_on
property). We already have a verbose extension namespace, but should consider a future where graphs isn't an extension any more but part of the core spec

[EwanC]

Closing as "won't do" for revision 1, can create a new issue if based on user feedback from revision 1 there are suggestions for improvement.