Fix: document: Fix there is no API Reference in clock

Add the doxygen group in clock rst file to add the API Reference
chapter. Fix: #37032

Signed-off-by: Jian Kang <jianx.kang@intel.com>

doc/reference/kernel/timing/clocks.rst

@@ -347,3 +347,9 @@ the time they passed the timeout to you.  Care must be taken to call
 this function just once, as synchronously as possible to the timeout
 creation in user code.  It should not be used on a "stored" timeout
 value, and should never be called iteratively in a loop.
+
+
+API Reference
+*************
+
+.. doxygengroup:: clock_apis

include/kernel.h

@@ -1515,6 +1515,7 @@ static inline void *z_impl_k_timer_user_data_get(const struct k_timer *timer)
 
 /**
  * @addtogroup clock_apis
+ * @ingroup kernel_apis
  * @{
  */