Document apps in doxygen module style, create more top-level categories to structure documentation

src/core/ip.c

@@ -44,6 +44,7 @@
  * @ingroup callbackstyle_api
  * 
  * @defgroup ipaddr IP address handling
+ * @ingroup infrastructure
  * 
  * @defgroup ip4addr IPv4 only
  * @ingroup ipaddr

src/core/memp.c

@@ -40,6 +40,8 @@
 
 /**
  * @defgroup mempool Memory pools
+ * @ingroup infrastructure
+ * Custom memory pools
  */
 
 #include "lwip/opt.h"

src/core/pbuf.c

@@ -5,6 +5,7 @@
 
 /**
  * @defgroup pbuf Payload buffers (PBUF)
+ * @ingroup infrastructure
  *
  * Packets are built from the pbuf data structure. It supports dynamic
  * memory allocation for packet contents or can reference externally