[libc] [llvm] [libc][docs] add redirect for math/index.html (PR #120274)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

We have a corporate policy on "externally managed python environments" that blocks the usage of `pip install` in favor of `apt install`; but I don't think `pip-compile` is packaged for debian FWICT?

https://github.com/llvm/llvm-project/pull/120274