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

[via llvm-commits]

uwendi wrote:

Thank you for your review @CarlosAndresRamirez and for all the small corrections that you helped me apply today. My last commit takes into consideration your last PR to update the WG's docs, to prevent a potential conflict that could cause losing your new section "Current Topics & Backlog". Everything should be OK now.

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