Add a Doxyfile and preliminary module-level docs.

src/mem.h

@@ -1,5 +1,13 @@
 #pragma once
 
+/**
+** \defgroup mem Memory management
+** tf_malloc() and friends use malloc() behind the scenes but optionally
+** track memory per system (OpenSSL, sqlite, libuv, ...) and store callstacks
+** to help debug leaks.
+** @{
+*/
+
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -42,3 +50,5 @@ typedef struct _tf_mem_allocation_t
 } tf_mem_allocation_t;
 
 tf_mem_allocation_t* tf_mem_summarize_allocations(int* out_count);
+
+/** @} */