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

doc/doxygen/lwip.Doxyfile

@@ -2080,6 +2080,7 @@ PREDEFINED             = NO_SYS=0 \
                          LWIP_IGMP=1 \
                          LWIP_TCP=1 \
                          LWIP_SNMP=1 \
+                         SNMP_USE_RAW=1 \
                          MEMP_OVERFLOW_CHECK=0 \
                          MEMP_SANITY_CHECK=1 \
                          LWIP_ARP=1 \

doc/doxygen/main_page.h

@@ -6,6 +6,12 @@
  * @defgroup threadsafe_api Thread-safe APIs
  * Thread-safe APIs, blocking functions. More overhead, but can be called
  * from any thread except TCPIP thread.
+ * 
+ * @defgroup infrastructure Infrastructure
+ * 
+ * @defgroup addons Addons
+ * 
+ * @defgroup apps Applications
  */
 
 /*! \mainpage lwIP Documentation

doc/doxygen/ppp.h

@@ -1,4 +1,5 @@
 /**
  * @defgroup ppp PPP
+ * @ingroup addons
  * @verbinclude "ppp.txt"
  */