Add a Doxyfile and preliminary module-level docs.

src/ssb.h

@@ -1,5 +1,11 @@
 #pragma once
 
+/**
+** \defgroup ssb SSB
+** Everything about SSB, SHS, and MUXRPC happens here.
+** @{
+*/
+
 #include "quickjs.h"
 
 #include <stdbool.h>
@@ -230,3 +236,5 @@ const char* tf_ssb_get_room_name(tf_ssb_t* ssb);
 void tf_ssb_set_room_name(tf_ssb_t* ssb, const char* room_name);
 
 void tf_ssb_schedule_work(tf_ssb_t* ssb, int delay_ms, void (*callback)(tf_ssb_t* ssb, void* user_data), void* user_data);
+
+/** @} */