[llvm] [QualGroup][docs] Reorganize QualGroup docs under Reference section (PR #160021)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> Hi @kbeyls Would you be so kind to help us merge this patch, if it looks ok to you? Thank you very much in advance.

Sure, I'll merge it once the pre-merge checks have run and come back green.
BTW, I guess you could ask for commit access yourself @uwendi, so that in the future you will no longer have to rely on me to do these merges?

https://github.com/llvm/llvm-project/pull/160021