Add Pixi material as extra material for Conda

[fasterius]

As discussed at the November 2024 course, we should add material for Pixi as a Conda replacement, but as extra material for the Conda tutorial. Possible as a whole separate page, but linked in the Conda extra material (but not in the schedule).

While Pixi seems to be fairly easy as a drop-in replacement, Conda is still heavily incorporated in so many other tools and infrastructures (Snakemake, Nextflow, nf-core, etc.) that fully replacing Conda with Pixi is a bit premature.