[MLGO][Docs] Add initial skeleton of MLGO docs
This adds an initial skeleton of the MLGO docs. This is intended to be
fleshed out over the next couple days as we work on filling out the
relevant sections on the tooling/features that are available in upstream
LLVM.
Reviewers: efriedma-quic, mtrofin
Reviewed By: mtrofin
Pull Request: https://github.com/llvm/llvm-project/pull/139177
