REF: nanpercentile -> array_algos.quantile
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

> What's the rationale for this? Currently all nan* functions live in nanops.py, which is a nice pattern?

these two functions are used exclusively in this file, and don't have any intra-nanops dependencies (e.g. _get_values, _wrap_results).  There's nothing _wrong_ with them being in nanops, but array_algos.quantile is better choice, and remains tightly-scoped. this is fine as it decouples the dependency structure a big. can you rebase rebased + greenish