Align functorch docs with PyTorch's (#85856)
This PR:
- changes the header/footer to be the same as PyTorch docs
- removes the functorch logo (we don't need it anymore, functorch has
been adopted into PyTorch)
- adjusts the functorch docs to make it clear that the page is functorch
documentation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85856
Approved by: https://github.com/svekars, https://github.com/samdow
