[llvm] [llvm][release] Add links to automatically built packages (PR #147719)

[David Spickett via llvm-commits]

DavidSpickett wrote:

ping!

Would be cool to get this (or https://github.com/llvm/llvm-project/pull/147021 which is a more limited version) into 21.x if possible, but of course up to you whether the risk with that is acceptable. I thought that the release candidate stage would be a good time to try this out before the first proper release.

https://github.com/llvm/llvm-project/pull/147719