[moe training] Update torchao docsite with MoE training docs

Currently the MoE training docs live in this [README](https://github.com/pytorch/ao/blob/main/torchao/prototype/moe_training/README.md). 

To make the prototype more discoverable and usable, we should:

1. Update the the [docsite](https://docs.pytorch.org/ao/stable/index.html)
2. Update torchtitan docs with examples for mxfp8 moe training
