Fix Nix manual links from search results

[lucperkins]

At the moment, links to the Nix manual in search results are broken. For example, if I search "nix.conf bash-prompt-prefix" in Google I get this link, which 404s:

https://nixos.org/manual/nix/stable/command-ref/conf-file

Removing /stable, however, yields a working link:

https://nixos.org/manual/nix/command-ref/conf-file

I'm not 100% sure that this is the right redirect approach in Netlify but I at least wanted to venture a possible solution.

[proofconstruction]

I love that we're all working on this in parallel. See https://github.com/NixOS/nixos-homepage/issues/1448 for more for additional context

[lucperkins]

@proofconstruction

[fricklerhandwerk]

Addressed downstream where I broke it: https://github.com/NixOS/nixos-homepage/pull/1451