[llvm] [llvm][docs] Add Approvals section to GitHub guide (PR #113434)

[David Spickett via llvm-commits]

DavidSpickett wrote:

@JL2210 based on another bit of your feedback.

I agree that the code review and github documents should ideally be one document, or perhaps one document for pr authors and one for pr reviewers. But for the moment, linking between them saves us repeating ourselves here.

(part of the reason there are two documents is for a while, GitHub was purely hosting and Phabricator was where reviews were done)

https://github.com/llvm/llvm-project/pull/113434