Add a Doxyfile and preliminary module-level docs.

src/ssb.db.h

@@ -1,5 +1,12 @@
 #pragma once
 
+/**
+** \defgroup ssb_db SSB Database
+** This is the main interface to SSB persistence.  Everything about getting and
+** storing messages and blobs goes through here.
+** @{
+*/
+
 #include "ssb.h"
 
 #include "quickjs.h"
@@ -66,3 +73,5 @@ tf_ssb_db_stored_connection_t* tf_ssb_db_get_stored_connections(tf_ssb_t* ssb, i
 void tf_ssb_db_forget_stored_connection(tf_ssb_t* ssb, const char* address, int port, const char* pubkey);
 
 int tf_ssb_sqlite_authorizer(void* user_data, int action_code, const char* arg0, const char* arg1, const char* arg2, const char* arg3);
+
+/** @} */