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

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

> I'm somewhat wondering if it'd be better to move the bit on branches and stacked PRs even further down, so that the basic workflow with these bits are sequentially at first, then diving into the bits of advanced PRs only later.

+1 Updated :) 

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