[doc] Link Hugging Face models to the ExecuTorch doc (#10154)

### Summary
ExecuTorch doc and repo have been successfully linked to the main entry
on [Hugging Face
doc](https://huggingface.co/docs/optimum/main/en/index), now we need to
link the Hugging Face doc and repo
([`huggingface/optimum-executorch`](https://github.com/huggingface/optimum-executorch))
to ExecuTorch doc for seamless experience.

### Test plan
Doc build on CI

Co-authored-by: Guang Yang <guangyang@fb.com>