update getting started guide and add llm gateway and prompt gateway samples (#330)

docs/source/guides/includes/arch_config.yaml

@@ -31,6 +31,7 @@ prompt_targets:
     endpoint:
       name: app_server
       path: /agent/summary
+      http_method: POST
     # Arch uses the default LLM and treats the response from the endpoint as the prompt to send to the LLM
     auto_llm_dispatch_on_response: true
     # override system prompt for this prompt target
@@ -41,6 +42,7 @@ prompt_targets:
     endpoint:
       name: app_server
       path: /agent/action
+      http_method: POST
     parameters:
       - name: device_id
         type: str