Set up AI usage guidelines

[Xuanwo]

More and more contributors, including myself, are using AI coding agents to contribute to OpenDAL. It’s helpful, but misusing LLMs can lead to poor responses for both contributors and maintainers. For example, discuss questions by pasting the response from AI agents, write verbose PR descriptions, or submit issues based on random AI findings.

So I think it's time we set up AI usage guidelines and add them to our contributing guide. In this way, we can guide our community to use AI in a responsible way.

[Xuanwo]

The DataFusion has a great example for us: https://github.com/apache/datafusion/pull/18237