Doc: nix-shell doc doesn't work (only nix develop .#nixpkgs-manual.shell works) and the redirects complains doc/build-helpers/fixed-point-arguments.chapter.md

[ShamrockLee]

Issue description

1. Both https://github.com/NixOS/nixpkgs/blob/master/doc/README.md and the Nixpkgs Manual build log is telling me to run nix-shell doc and run redirects <identifier> <path> after adding new documentation anchors, and doc/README.md also sais that nix-shell doc will provide a devmode command. None of the commands presented after invoking nix-shell doc.
2. When I finally work around the above issue with nix develop .#nixpkgs-manual.shell, I typed redirects add-content ex-build-helpers-mkLocalDerivation-migration doc/build-helpers/fixed-point-arguments.chapter.md, and got OSError: [Errno 30] Read-only file system: '/nix/store/nhx2zqvzxr10wzcn6zgx0y7ynzcgag7w-source/doc/redirects.json'.
3. Including the redirects-related instructions in the build error message is helpful but doesn't explain the <path> to use. The help message of redirects offers nothing but the available commands and the names of their parameters, which is not very helpful.

Cc:
@GetPsyched (author of #353513)
@NixOS/documentation-team

Steps to reproduce

1. cd to the Nixpkgs code base.
2. Check out my feature branch for lib.extendMkDerivation: init #234651.
3. Run nix-shell doc at the root of the Nixpkgs code base.
4. Run type redirects and see error.
5. Run type devmode and see error.
6. Run nix develop .#nixpkgs-manual.shell get into the shell and get the above two commands.
7. Run redirects add-content ex-build-helpers-mkLocalDerivation-migration doc/build-helpers/fixed-point-arguments.chapter.md
8. See the following error messages:

Added new identifier: ex-build-helpers-mkLocalDerivation-migration
Traceback (most recent call last):
  File "/nix/store/74bkxpm7plcajxpn9ayhghwfgv0ddjhd-nixos-render-docs-redirects-0.0/bin/.redirects-wrapped", line 9, in <module>
    sys.exit(main())
             ^^^^^^
  File "/nix/store/74bkxpm7plcajxpn9ayhghwfgv0ddjhd-nixos-render-docs-redirects-0.0/lib/python3.12/site-packages/nixos_render_docs_redirects/__init__.py", line 112, in main
    with open(args.file, "w") as file:
         ^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 30] Read-only file system: '/nix/store/nhx2zqvzxr10wzcn6zgx0y7ynzcgag7w-source/doc/redirects.json'

Technical details

[GetPsyched]

Thanks for reporting, I had this issue during development but it was resolved before merger; looks like some edge case was missed. I'm looking into this

[magneticflux-]

I got this same error trying to work from the main nixpkgs directory. Moving to the nixpkgs/doc directory and running nix-shell worked as expected.

[fricklerhandwerk]

@ShamrockLee thanks for reporting, fixed 1 and 2 in #375194. Please open a separate issue for 3, this needs a bit more deliberation.