Add a Doxyfile and preliminary module-level docs.

src/ssb.import.h

@@ -1,6 +1,16 @@
 #pragma once
 
+/**
+** \defgroup tf_import Tilde Friends Import
+** Support for importing Tilde Friends apps from disk to the database.
+**
+** I don't know why it has SSB in the name.
+** @{
+*/
+
 typedef struct _tf_ssb_t tf_ssb_t;
 
 void tf_ssb_import(tf_ssb_t* ssb, const char* user, const char* path);
 void tf_ssb_import_from_zip(tf_ssb_t* ssb, const char* zip_path, const char* user, const char* path);
+
+/** @} */