[llvm] [docs][mlgo] Document `MLModelRunner` (PR #139205)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

After this was pointed to in the LLVM weekly, I thought to have a read of this document.
It seems to me that this document cannot be found through any of the indexing links under https://llvm.org/docs.
Maybe it would be useful to add a pointer to it in the section https://llvm.org/docs/Reference.html#additional-topics?

https://github.com/llvm/llvm-project/pull/139205