[Documentation] [BE] Add docs for MXFP8 training on Blackwell

[danielvegamyhre]

We have float8 docs, we should add mxfp8 docs as well, especially since we have a public blog post on accelerating training with torchtitan mxfp8 training: https://pytorch.org/blog/accelerating-2k-scale-pre-training-up-to-1-28x-with-torchao-mxfp8-and-torchtitan-on-crusoe-b200-cluster/