Proposal to modify the contributing guidelines and add AGENTS.md to dissuade AI contributions

Due to the infestation of AI slop commits on GitHub recently, I think it may be a good idea to add similar guidelines that other repositories (such as [MonoGame](https://github.com/MonoGame/MonoGame/blob/develop/AGENTS.MD)) have taken towards AI contributions.

While Silk.NET has seemingly been quite safe from these "contributions" so far, I feel it is only a matter of time before it is "discovered" by them and the "contributions" get sent this way.

As such it may be wise to get ahead of them before anything happens.