docs(config): add examples for config agents

PiperOrigin-RevId: 793871778

contributing/samples/sub_agents/root_agent.yaml

@@ -0,0 +1,11 @@
+# yaml-language-server: $schema=https://raw.githubusercontent.com/google/adk-python/refs/heads/main/src/google/adk/agents/config_schemas/AgentConfig.json
+name: root_agent
+model: gemini-2.0-flash
+description: Root agent
+instruction: |
+  If the user query is about life, you should route it to the life sub-agent.
+  If the user query is about work, you should route it to the work sub-agent.
+  If the user query is about anything else, you should answer it yourself.
+sub_agents:
+  - config_path: ./work_agent.yaml
+  - code: sub_agents.life_agent.agent