DOC: parallelize the number of Sphinx jobs by default
I'm assuming most people building docs would want that to happen as quickly as possible, so this has Sphinx use the maximum parallelism possible by default. [Corresponding Sphinx documentation.](https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-j)

- [ ] ~~closes #xxxx~~
- [x] tests ~~added~~ / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~
