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

[via llvm-commits]

uwendi wrote:

Hi @petar-jovanovic Could you kindly take a moment to review this PR at your earliest convenience? Your expertise and approval would be greatly appreciated, and if everything looks good, could you please proceed with merging it? Thank you so much for your assistance!

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