Recommend pixi for environment set-up

[Robinlovelace]

Currently we recommend micromamba as an environment manager: https://github.com/geocompx/geocompy?tab=readme-ov-file#install-dependencies-with-a-package-manager

I suggest we replace that with pixi, who provide 7 reasons for switching: https://prefix.dev/blog/pixi_a_fast_conda_alternative

It will have some cost to do so but this really does seem like the most future proof and least invasive way of managing environmennts, just tested it to install Python and it worked a charm:

pixi add python
pixi shell
python --version
# Python 3.12.4

It is so fast and efficient!

[Robinlovelace]

Closing for now, a bit out of scope, we can re-open later if needs be.