[ExecuTorch][llm] Add MoE quantized custom op documentation (#21539)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: https://github.com/pytorch/executorch/pull/21125 by
@digantdesai
^ Please use this as the source of truth for the PR details, comments,
and reviews
ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/digantdesai/75/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/75/head
Merge bot PR base:
https://github.com/pytorch/executorch/tree/gh/digantdesai/74/orig
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/75/orig
Differential Revision:
[D102382001](https://our.internmc.facebook.com/intern/diff/D102382001/)
@diff-train-skip-merge

Co-authored-by: Digant Desai <digantdesai@meta.com>