Merge #6750

6750: Remove documentation query, move doc handling to attributes r=matklad a=Veykril

Fixes #3182

Removes the documentation query in favor of `Attrs::docs`. Attrs already handlded doc comments partially but the alloc saving check was wrong so it only worked when other attributes existed as well. Unfortunately the `new` constructor has to do an intermediate allocation now because we need to keep the order of mixed doc attributes and doc comments.

I've also partially adjusted the `hover` module to have its tests check the changes, it still has some `HasSource` trait usage due to the `ShortLabel` trait usage, as that is only implemented on the Ast parts and not the Hir, should this ideally be implemented for the Hir types as well?(would be a follow up PR of course)

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>