treewide: Fix AST of Nix in all markdown files

[dasJ]

Description of changes

This PR does 2 things:

• It adds nix-fencing to nix code that had no syntax specifier
• It fixes all Nix ASTs to be parsable at least by nixfmt

The end-goal of this PR is to make the ASTs parsable by the formatter to get RFC-166 into the manual. This is important since the manual is likely to be one of the first contact points of new users and also a place where code is likely to be copied from. This might also make the code compliant with other parsers as well, allowing for future functionality like checking for renamed options.

As outlined above, this does not reformat the code with nixfmt (yet). This is the preparation to do such things in the future. There are however a lot of manual reformats to make the code look "normal" (like when adding {} around something that has to be indented now).

I am also aware that there are other places where documentation can be found, which are not in .md files. There's toPretty and the examples from the manual but the PR is probably big enough as-is.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[dasJ]

Since I mentioned this multiple times in the description: This is what the manual would look like if we nixfmt'ed it: helsinki-systems@8c506ddd8436

[fricklerhandwerk]

Nice! You may want to note in the commit message why we're using angle brackets there.

[DanielSidhion]

Neat! Thank you for doing this. Do you have anything documented on how you did this? I'm assuming you built a tool to do this work. Is it available anywhere? I'd like to add this as part of the documentation team's toolset.

[dasJ]

Changed the commit message that @fricklerhandwerk suggested and also linked Robert's comment there.

@DanielSidhion partly. I wrote a script that finds the issues, the fixing of the 600+ issues was a manual process. The script does not exist anymore because it has since grown into a full utility to analyze the nix formatting in markdown files and to automatically fix it if needed. I plan to upstream this along a full reformat (see #299554 (comment)) and a GitHub workflow that ensures that people don't accidentially break it again :)

[drupol]

impressive work! Cool !