feat: add pixi as a way to install package

[nichmor]

Checklist

• [ ] Added a news entry

[baszalmstra]

Lowering the barrier to be able to try a particular piece of software seems very vital to me. We want people to use this! I feel it's very useful to have a shortcut on how to use a package with your favorite package manager in the feedstock itself.

But I do agree it clutters up the readme a little. Perhaps we can add all package managers as foldable sections like this:

How to use

With Conda

``` conda env create .. ```

With Mamba

``` mamba install .. ```

With Pixi

``` pixi add .. ```

Removing mamba seems strange to me, in that sense we could also argue to remove the conda commands. 🤷