doc: update ffi design

assets/c-friendly-ffi-design/index.md

@@ -10,6 +10,7 @@ to write the accompanying C/C++ header files.
 
 * [Rust Side](rust-side.md)
     - [FFI Function Signature](rust-side/func-signature.md)
+    - [Library Architecture](rust-side/library-architecture.md)
     - [Allowed Data Types](rust-side/data-types.md)
     - [FFI Example](rust-side/example.md)
     - [Miscellaneous Stuff](rust-side/misc.md)