docs: add instructions for installing on Alpine Linux

[orhun]

I created the Alpine package for pixi: https://pkgs.alpinelinux.org/packages?name=pixi

This PR aims to add the instructions for installing pixi on Alpine.

Also, I made some formatting changes and added repology badge.

[ruben-arts]

Thanks!

[benglewis]

From what I can see, the pixi build for Alpine Linux does not work when installing anything :/

I get the following error:

(linux/arm64/v8) and no specific platform was requested
/ # pixi init
✔ Initialized project in /.
/ # pixi add python
  × An activation error occurred: IoError(Os { code: 2, kind: NotFound, message: "No such file or
  │ directory" })

/ #