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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Updating the hashes in #120315.

The bot will still be broken, but the fix is either:
- manual intervention
- `pip install -r llvm/docs/requirements.txt` BEFORE running `ninja <whatever to build docs>`.

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