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

[Mircea Trofin via llvm-commits]

mtrofin 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?

It's currently there under hidden topics, we'll move it out in a bit - wanted to finish the current batch of docs first.

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