[llvm] [QualGroup] Add August and September 2025 slides and unify Meeting Materials (PR #156897)

[Nikita Popov via llvm-commits]

nikic wrote:

@llvm/infrastructure-area-team Maybe the infrastructure team can provide some input on where to host your slides. I'm pretty sure it should be in llvm-www rather than llvm-project, but I can't comment on the specifics.

https://github.com/llvm/llvm-project/pull/156897