docs: Add AI Usage Policy and Guidelines to CONTRIBUTING.md

[Ayushmore1214]

What would you like to be added: As discussed in community meet, we need a simple AI policy in our CONTRIBUTING.md to protect maintainer time and keep our communication human, Adding a clean guide on when AI is okay and when it's not

Why is this needed:

[Ayushmore1214]

https://github.com/backstage/backstage/blob/master/CONTRIBUTING.md#ai-use-policy-and-guidelines for context on what we are trying to propose

[ObaidAbdullah16]

I’d like to work on this if nobody is already. As I am still new to open source, I do use AI quite a bit for things like understanding the codebase, summarizing docs, and getting context around issues, so this topic feels pretty relevant to me as well.

One thing I especially liked in the Backstage guidelines was the focus on not wasting maintainer time. I think it is the most important part for this policy

My plan would be to first go through the current contribution flow and the Backstage guidelines, then try drafting a lightweight policy focused on contribution quality, understanding and testing changes, and clear communication rather than restricting AI usage itself.

[Ayushmore1214]

Sure @ObaidAbdullah16 you can go ahead assigning it to you!