Add a Doxyfile and preliminary module-level docs.

src/ssb.export.h

@@ -1,5 +1,15 @@
 #pragma once
 
+/**
+** \defgroup tf_export Tilde Friends Export
+** Support for exporting Tilde Friends apps from the database to disk.
+**
+** I don't know why it has SSB in the name.
+** @{
+*/
+
 typedef struct _tf_ssb_t tf_ssb_t;
 
 void tf_ssb_export(tf_ssb_t* ssb, const char* key);
+
+/** @} */