libc++ 17 release notes link is broken on https://releases.llvm.org

The link is pointing to https://releases.llvm.org/17.0.1/projects/libcxx/docs/ReleaseNotes.html, however the actual libc++ release notes are at https://releases.llvm.org/17.0.1/projects/libcxx/docs/ReleaseNotes/17.html.

We could either fix the link here or libc++ could maintain a stable `ReleaseNotes.html` page that points to whatever the "current" release notes are.