download: add installation instructions for pixi

[gregorywaynepower]

Adds installation instructions for pixi users for the following usecases:

• adding as a globally available tool (no more having to activate environments)
• adding as part of a project
• adding instructions for interoperability for conda users

[wenzeslaus]

It is okay or even appropriate to say why one would choose the given installation method, but I don't think we should be teaching the tools - talking mostly about the conda exchange part. Why it is there?

[gregorywaynepower]

I thought it was appropriate since there's other content about how to create an environment.yaml with conda. I'm assuming that folks are familiar with conda and I don't want users to assume the pixi.toml format is compatible with environment.yaml.

[neteler]

IMHO compilation/installation details would rather go to
https://grasswiki.osgeo.org/wiki/Compile_and_Install

[gregorywaynepower]

IMHO compilation/installation details would rather go to

https://grasswiki.osgeo.org/wiki/Compile_and_Install

pixi could be used for contributors since hugo is on conda-forge. Unfortunately hugo version 0.113.0 isn't available for Windows users. But I'd put that in a separate PR.

[gregorywaynepower]

@wenzeslaus Would it be better to create new content headings for:

Installation

• conda
• mamba
• pixi
• (sub heading for global installation)
• (sub heading for part of project/workspace)

(remove the section on environment.yaml files)

Usage

• python
• command line
• (sub heading for conda)
• (sub heading for pixi for global installation)