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

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis approved this pull request.

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".

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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.

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