docs: add contributing guidelines for dev setup using pixi #3414
Description
Fantastic work!! One final thing: can you please update the testing guidelines here: https://snakemake.readthedocs.io/en/stable/project_info/contributing.html#testing-guidelines (i.e. in the corresponding place under docs/), to explain how to setup and run the tests just using pixi?
Originally posted by @johanneskoester in #3369 (review)