Add an LLM policy for rust-lang/rust

No comment on this PR may mention the following topics: • Long-term social or economic impact of LLMs • The environmental impact of LLMs • Anything to do with the copyright status of LLM output • Moral judgements about people who use LLMs We have asked the moderation team to help us enforce these rules.

– Add an LLM policy for rust-lang/rust, GitHub