[infra-if] fix documentation group order (#12823)

This commit moves the `@addtogroup plat-infra-if` Doxygen block to the
top of the `infra_if.h` header file. Previously, it was defined
after `otPlatInfraIfLinkLayerAddress`, causing that structure and
the `OT_PLAT_INFRA_IF_MAX_LINK_LAYER_ADDR_LENGTH` macro to be excluded
from the `plat-infra-if` module in the generated documentation.

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (587)
+#define OPENTHREAD_API_VERSION (588)
 
 /**
  * @addtogroup api-instance

include/openthread/platform/infra_if.h

@@ -47,6 +47,15 @@
 extern "C" {
 #endif
 
+/**
+ * @addtogroup plat-infra-if
+ *
+ * @brief
+ *   This module includes the platform abstraction for the adjacent infrastructure network interface.
+ *
+ * @{
+ */
+
 #define OT_PLAT_INFRA_IF_MAX_LINK_LAYER_ADDR_LENGTH 16 ///< Maximum InfraIf Link-layer address length.
 
 /**
@@ -58,15 +67,6 @@ typedef struct otPlatInfraIfLinkLayerAddress
     uint8_t mLength;                                               ///< The address length (number of bytes).
 } otPlatInfraIfLinkLayerAddress;
 
-/**
- * @addtogroup plat-infra-if
- *
- * @brief
- *   This module includes the platform abstraction for the adjacent infrastructure network interface.
- *
- * @{
- */
-
 /**
  * Tells whether an infra interface has the given IPv6 address assigned.
  *