sys: doc: pipe.h spams the manual

This PR adds a new group for the pipe module, so the .h file does not
spam the manual in Modules/System.

sys/include/pipe.h

@@ -17,7 +17,8 @@
  */
 
 /**
- * @addtogroup  sys
+ * @defgroup    sys_pipe Pipe IPC
+ * @ingroup     sys
  * @{
  * @file
  *

sys/pipe/pipe.c

@@ -17,7 +17,7 @@
  */
 
 /**
- * @ingroup     sys
+ * @ingroup     sys_pipe
  * @{
  * @file
  * @brief       Implementation for statically allocated pipes.

sys/pipe/pipe_dynamic.c

@@ -17,7 +17,7 @@
  */
 
 /**
- * @ingroup     sys
+ * @ingroup     sys_pipe
  * @{
  * @file
  * @brief       Implementation for dynamically allocated pipes.