doc: json: add to doxygen group

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

include/json.h

@@ -7,6 +7,20 @@
 #ifndef __JSON_H
 #define __JSON_H
 
+
+
+/**
+ * @brief Structured Data
+ * @defgroup structured_data Structured Data
+ */
+
+
+/**
+ * @defgroup json JSON
+ * @ingroup structured_data
+ * @{
+ */
+
 #include <misc/util.h>
 #include <stddef.h>
 #include <zephyr/types.h>
@@ -528,4 +542,7 @@ int json_obj_encode(const struct json_obj_descr *descr, size_t descr_len,
 		    const void *val, json_append_bytes_t append_bytes,
 		    void *data);
 
+/**
+ * @}
+ */
 #endif /* __JSON_H */