[llvm] [NFC][AMDGPU][Doc] Add developer guideline (PR #194425)

[Shilei Tian via llvm-commits]

shiltian wrote:

The motivation of this document is to provide a single place to highlight points we've had to repeatedly call out during code reviews. While many of these overlap with general LLVM documentation, that information is spread across different places, often quite long, and not always clearly enforced in the code, so it's easy to miss.

I don't treat documentation the same as code, so some redundancy is fine, especially for points that come up frequently in reviews. I can add references to the relevant LLVM docs where appropriate, but I still think it's worth keeping these here.

https://github.com/llvm/llvm-project/pull/194425