[llvm] [docs] Refactor QualGroup.rst: add membership rules, restructure sections (PR #157804)

[Carlos Andres Ramirez via llvm-commits]

https://github.com/CarlosAndresRamirez commented:

Changes look good to me.
Let's see if we can get this approved by someone with the right access in the repository.

https://github.com/llvm/llvm-project/pull/157804