add `pytest-doctestplus` for doctest-only option

* [`failing doctests`](https://github.com/carsonfarmer/fastpair/actions/runs/9882974279/job/27296839534?pr=72#step:6:99) due to the new formatting in `numpy` -- only run doctests on one CI environment 
* add [`pytest-doctestplus`](https://github.com/scientific-python/pytest-doctestplus) plugin for option to run only doctests through `pytest`