[llvm] [DevPolicy] Add guidance to not submit code or reviews on someone else's (PR #69701)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

Thank you everyone who provided feedback here and offline.
The Code of Conduct committee evaluated all feedback and held more conversations with people.
The Code of Conduct committee based on that feedback and evaluation proposes the updated text that is part of the commit that was added to this PR just now [#273db04](https://github.com/llvm/llvm-project/pull/69701/commits/273db042892a5087841c4b880be8bbde3e86cc7f)

https://github.com/llvm/llvm-project/pull/69701