docs: add new diagrams for game loop, gateway, guarded suspension, half-sync/half-async, health check

gateway/README.md

@@ -35,6 +35,10 @@ Wikipedia says
 
 > A server that acts as an API front-end, receives API requests, enforces throttling and security policies, passes requests to the back-end service and then passes the response back to the requester.
 
+Sequence diagram
+
+![Gateway sequence diagram](./etc/gateway-sequence-diagram.png)
+
 ## Programmatic Example of Gateway Pattern in Java
 
 First, we define a `Gateway` interface. This interface represents the contract for our external services. Each service that we want to interact with will implement this interface.