refactor(mstsgu): follow up to PR 913 (#920)

- Update tokio-tungstenite to latest
- Fix the dependencies
- Move the top-level documentation into a README.md that we re-include
in the source code
- Re-order the imports using the nightly formater
- Audit the unwraps and remove them
- Fix the UTF-16 string length computation

Author: CBenoit

ARCHITECTURE.md

@@ -256,6 +256,10 @@ State machines to drive an RDP connection acceptance sequence
 
 Extendable skeleton for implementing custom RDP servers.
 
+#### [`crates/ironrdp-mstsgu`](./crates/ironrdp-mstsgu) (@steffengy)
+
+Terminal Services Gateway Server Protocol implementation.
+
 #### [`crates/ironrdp-glutin-renderer`](./crates/ironrdp-glutin-renderer) (no maintainer)
 
 `glutin` primitives for OpenGL rendering.