[llvm] [docs] Update docs on code-review process (PR #111735)

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

My bad for not noticing https://llvm.org/docs/CodeReview.html, I didn't realise there were two documents (though https://llvm.org/docs/Contributing.html#how-to-submit-a-patch felt weirdly unfamiliar).

I've reverted the changes in Contributing.rst and instead added a paragraph to CodeReview.rst.

https://github.com/llvm/llvm-project/pull/111735