[BE] Dockerize PyTorch docs jobs (#100601)
Saw some connection error to pip in docs jobs today, so let's dockerize it:
* https://github.com/pytorch/pytorch/actions/runs/4877612277/jobs/8702572072
* https://github.com/pytorch/pytorch/actions/runs/4877612277/jobs/8702572072
Some additional fixes:
* Moving the docs script from under `.circleci` to under `.ci` as they should be
* Linter (as scripts under .ci are subjected to shellcheck)
* Fix some minor Sphinx warnings in functorch docs
### Testing
Docs previews look fine:
* https://docs-preview.pytorch.org/100601/index.html
* https://docs-preview.pytorch.org/100601/cppdocs/index.html
* https://docs-preview.pytorch.org/100601/functorchdocs/index.html
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100601
Approved by: https://github.com/clee2000
