Nix language primer with `shell.nix`

The Learning Journey WG identified that the second tutorial in the learning journey should be a tutorial walking the user through creating a shell.nix. This is the tracking issue for discussion on this tutorial and related work.

• Compile a survey of shell.nix tutorials (@khaled)
• Consider if there are elements of existing tutorials that we should include
• Prepare an outline of the tutorial
• Prepare a draft of the tutorial
• Submit the draft as a PR

Tutorial contents

• Provide the reader with a small, functioning shell.nix installing Python
• Explain that once you've experimented with packages via nix-shell -p you can enter the same environment repeatedly by declaring it in a shell.nix file.
• You can also use shell.nix to configure other aspects of the environment:
  • Environment variables
  • Shell hooks
  • Shell prompts
• Illustrate how to add packages to the shell, explaining the basic differences between buildInputs and packages
• This shell.nix will just grab nixpkgs from <nixpkgs>, leaving pinning, channels, etc to later tutorials

How

• Where to put the result: A new file in https://github.com/NixOS/nix.dev/tree/master/source/tutorials
• Person to contact for help:
  • Technical: @infinisil
  • Editorial: @zmitchell