doc: add kernel version API to doxygen

Put kernel version API into doxygen and make it available as a
documented API.

Fixes #6319

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/api/kernel_api.rst b/doc/api/kernel_api.rst
index 384d63e..2abf50a 100644
--- a/doc/api/kernel_api.rst
+++ b/doc/api/kernel_api.rst
@@ -241,6 +241,14 @@
    :project: Zephyr
    :content-only:
 
+Kernel Version
+**************
+Kernel version handling and APIs related to kernel version being used.
+
+.. doxygengroup:: version_apis
+   :project: Zephyr
+   :content-only:
+
 Memory Domain
 *************