Improve docs

docs/how-to-guides/using-service.md

@@ -0,0 +1,14 @@
+# How to use a GreatAI service
+
+After [creating a GreatAI service](/how-to-guides/cerate-service) by wrapping your prediction function, it's time to do some prediction.
+
+Let's use the following example:
+
+```python "type_safe_greeter.py"
+from great_ai import GreatAI
+
+@GreatAI.create
+def type_safe_greeter(your_name: str) -> str:
+    return f'Hi {your_name}'
+```
+