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

[via llvm-commits]

uwendi wrote:

@nikic following up on your earlier feedback about slide hosting. This PR adds the August and September 2025 slides and merges the Meeting Minutes and Slides sections into a unified “Meeting Materials” section.

Slides are still temporarily stored under `llvm/docs/qual-wg/slides/`, but I’ve added a note that we’ll migrate them to `llvm-www` once the appropriate location is confirmed. Would appreciate your thoughts on whether creating a subfolder `qual-wg` in `llvm-www` would be suitable for that purpose.

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