[llvm] [llvm][docs] Clean up the "Landing Your Change" section of the GitHub docs (PR #112869)

[David Spickett via llvm-commits]

DavidSpickett wrote:

@JL2210 This may address some of your feedback on the doc. I will look at the doc as a whole in another PR.

https://github.com/llvm/llvm-project/pull/112869