Add a Doxyfile and preliminary module-level docs.

src/tls.h

@@ -1,5 +1,11 @@
 #pragma once
 
+/**
+** \defgroup tls TLS
+** A minimal wrapper around OpenSSL.
+** @{
+*/
+
 #include <stdbool.h>
 #include <stddef.h>
 
@@ -39,3 +45,5 @@ int tf_tls_session_write_plain(tf_tls_session_t* session, const char* buffer, si
 int tf_tls_session_read_encrypted(tf_tls_session_t* session, char* buffer, size_t bytes);
 int tf_tls_session_write_encrypted(tf_tls_session_t* session, const char* buffer, size_t bytes);
 bool tf_tls_session_get_error(tf_tls_session_t* session, char* buffer, size_t bytes);
+
+/** @} */