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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> I like the new order better because it moves an advanced topic later on in the guide, but also lays out "Stacked Pull Requests" after "Pull Requests".
> 
> I also can't help but think of this minor tweak too:
> 
> * Branches
> * Pull Requests
> * Stacked Pull Requests
> 
> On one hand the above order feels slightly more natural to me. But on the other hand the Branches section is very brief ATM, and is mostly tied to stacked PRs.

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.

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.
- Before your first PR
- Pull Requests
- Approvals
- Landing your change

But the suggested order is clearly better than what we have right now anyway.

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