Lib: SMF: Add Doxygen commands.

Added Doxygen markup.

Signed-off-by: Glenn Andrews <glenn.andrews.42@gmail.com>

doc/services/smf/index.rst

@@ -542,3 +542,10 @@ The statechart for this test is below.
       STATE_B -> STATE_C
       STATE_C -> STATE_D
    }
+
+
+
+API Reference
+*************
+
+.. doxygengroup:: smf

include/zephyr/smf.h

@@ -4,11 +4,22 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-/* State Machine Framework */
+/**
+ * @file
+ *
+ * @brief State Machine Framework header file
+ */
 
 #ifndef ZEPHYR_INCLUDE_SMF_H_
 #define ZEPHYR_INCLUDE_SMF_H_
 
+/**
+ * @brief State Machine Framework API
+ * @defgroup smf State Machine Framework API
+ * @ingroup os_services
+ * @{
+ */
+
 #ifdef CONFIG_SMF_ANCESTOR_SUPPORT
 /**
  * @brief Macro to create a hierarchical state.
@@ -187,4 +198,8 @@ int32_t smf_run_state(struct smf_ctx *ctx);
 }
 #endif
 
+/**
+ * @}
+ */
+
 #endif /* ZEPHYR_INCLUDE_SMF_H_ */