Add extent observers to common group interface

rank(), rank_dynamic() and static_extent() are all exposed as static
member functions in mdspan, so we follow that precedent here.

adoc/extensions/sycl_khr_group_interface.adoc

@@ -65,6 +65,10 @@ class GroupT {
 #if defined (__cpp_lib_mdspan)
   constexpr extents_type extents() const noexcept;
   constexpr index_type extent(rank_type r) const noexcept;
+
+  static constexpr rank_type rank() noexcept;
+  static constexpr rank_type rank_dynamic() noexcept;
+  static constexpr size_t static_extent(rank_type r) noexcept;
 #endif
   constexpr size_type size() const noexcept;
 
@@ -139,6 +143,36 @@ _Returns_: The number of work-items in the specified dimension of the group.
 
 '''
 
+.[apidef]#GroupT::rank#
+[source,role=synopsis,id=api:khr-group-interface-common-group-rank]
+----
+static constexpr rank_type rank() noexcept;
+----
+
+_Effects_: Equivalent to [code]#return extents_type::rank();#.
+
+'''
+
+.[apidef]#GroupT::rank_dynamic#
+[source,role=synopsis,id=api:khr-group-interface-common-group-rank_dynamic]
+----
+static constexpr rank_type rank_dynamic() noexcept;
+----
+
+_Effects_: Equivalent to [code]#return extents_type::rank_dynamic();#.
+
+'''
+
+.[apidef]#GroupT::static_extent#
+[source,role=synopsis,id=api:khr-group-interface-common-group-static_extent]
+----
+static constexpr size_t static_extent(rank_type r) noexcept;
+----
+
+_Effects_: Equivalent to [code]#return extents_type::static_extent(r);#.
+
+'''
+
 .[apidef]#GroupT::size#
 [source,role=synopsis,id=api:common-group-size]
 ----