[llvm] [llvm][docs] Reorder sections in GitHub.rst (PR #134212)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

> I'd prefer this order here, keeping Branches later, as they're not essential for the basic workflow, they're only needed for stacked PRs.

Just clarifying that I too agree with this order: simple to advanced topics. 👌

https://github.com/llvm/llvm-project/pull/134212