Restructure documentation. Create two top-level sections for thread-safe and callback-style APIs.

src/api/tcpip.c

@@ -441,6 +441,7 @@ tcpip_trycallback(struct tcpip_callback_msg* msg)
 }
 
 /**
+ * @ingroup lwIP
  * Initialize this module:
  * - initialize all sub modules
  * - start the tcpip_thread