DOC: Loading sphinxcontrib.spelling to sphinx only if it's available (#21396)
- [X] closes #21396
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think the optional dependencies go directly into the `.txt` files for pip and conda, and not into the yaml file. Please correct me if I'm wrong.

Seems like besides `sphinxcontrib-spelling`, a binary program `enchant` is required, and it's not available in conda for what I've seen. Not sure what's the right approach for it.