Declarative reproducible developer environments

[worktheclock]

RITZA language edit.

[fricklerhandwerk]

Sorry it took so long for a review.

This is really good, thank you very much!

PS: Please do the CLA thing so I can merge this.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-06-26-documentation-team-meeting-notes-58/29666/1

[proofconstruction]

I guess we have a setting to automatically request review when force pushing or something?

Anyway, I rebased* the original commit onto master and made several mostly-small changes to the prose as well as larger changes to the way some things are discussed, in particular:

• more information in the introduction describing why this tutorial differs from the ad-hoc developer environments,
• better explaining why the declarative approach is (more) useful,
• better motivating the shell.nix customization, and
• using more precise language to describe what's happening

*- I hope I did this correctly; I can't tell whether the first commit by the original author was preserved, or if I accidentally included some of my changes to the file that got committed there. Apologies if so.

[fricklerhandwerk]

Looks great, awesome work!