Update examples docs with realtime section (#1113)

## Summary
- document realtime examples folder

## Testing
- `make format`
- `make lint`
- `make mypy`
- `make tests`

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https://chatgpt.com/codex/tasks/task_i_687581e2bd94832498132d4a961e02d4

Author: jhills20

docs/examples.md

@@ -40,3 +40,6 @@ Check out a variety of sample implementations of the SDK in the examples section
 
 - **[voice](https://github.com/openai/openai-agents-python/tree/main/examples/voice):**
   See examples of voice agents, using our TTS and STT models.
+
+- **[realtime](https://github.com/openai/openai-agents-python/tree/main/examples/realtime):**
+  Examples showing how to build realtime experiences using the SDK.