Add docstring example for dynamic_batching decorator

[veeceey]

Summary

• Adds comprehensive docstring example for @dynamic_batching decorator
• Previously had TODO(johannes) to add example
• Demonstrates proper usage with Executor and requests decorator

Changes

• Added example showing:
  • Using @dynamic_batching with @requests decorator
  • Setting preferred_batch_size and timeout parameters
  • Processing batched DocumentArray in async method
• Removed TODO comment as example is now complete

Test plan

• Example follows existing patterns from integration tests
• Documentation is clear and follows Jina docstring conventions
• No code changes, only documentation enhancement

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.