NixOS manual improvements

[aanderse]

It would be nice if someone could rewrite the abstractions section of the NixOS manual to use something other than services.httpd for examples. See #73113 for the primary motivation I have on this.

[flokli]

As discussed with @aanderse on IRC, the httpd submodule probably isn't a good candidate anymore, as a lot of the boilerplate has been removed.

The abstractions section could however showcase composing multiple declarative nixos containers with similar configuration.

[aanderse]

ping @infinisil in case he has some ideas ❤️

[infinisil]

Hm, I don't actually know of any example where I had to abstract like that in my config. Seems like most NixOS modules already do a pretty good job at being non-repetitive. So idea: How about removing that section, and instead writing a section on abstractions in Nix in general? The abstractions presented there aren't module system specific anyways.

[aanderse]

That's a reasonable idea too. Would you imagine that new section to be in the NixOS manual as well, or the Nix manual?

[infinisil]

Probably in the Nix manual, it would fit nicely inbetween 15.3 and 15.4