Add documentation for common flake outputs

[Radvendii]

First attempt at addressing #5716

Some issues with this:

1. Kind of verbose.
   • it would be nice if there were a standard conversion between singular / plural as discussed in Rename defaultX flake outputs to X.default #5532, as the documentation could be half as long.
   • I'm not sure if this is the right place for all the information that it contains. maybe we want something more slimmed down with references to where to find the rest of the information (for instance, apps could just have a link to the nix run page, and not tell people how nix run works.
2. Bundler interface is in flux right now, so that section is wating on bundle: pass drv attrset instead of path #5456
3. I've never used Hydra, so I don't know what to write for the hydraJobs section.
4. I'm worried that this will become out of date very quickly. How are we going to make sure it gets kept up to date?

[edolstra]

To prevent it from becoming outdated, maybe it should be restricted to flake outputs that are used/recognized by nix itself (including ones that are checked by nix flake check, like nixosModules).

The manpage for nix flake check contains some of this info already in the "Evaluation checks" section, so maybe we should merge that section into this list to avoid redundancy.

[edolstra]

This doesn't format in a nice way, namely "Used-by: nix flake check Checked: Yes Type: systemAttrs (attrsOf (derivation))".

[tomberek]

• cross-reference with templates repo

[Ericson2314]

Formally marking as draft, given old title.

[iFreilicht]

Oh my, I've been searching for something like this for weeks! The best I could find is the "Output Schema" section on the NixOS Wiki page on Flakes.

To prevent it from becoming outdated, maybe it should be restricted to flake outputs that are used/recognized by nix itself (including ones that are checked by nix flake check, like nixosModules).

That might help with preventing outdated info, but it would lead to the same problem we already have, which is that documentation is split across many different locations. Of course, the specifics of nixosConfigurations should be documented in the NixOS wiki, but having all "well-known" output names listed here would be very useful for discoverability. The PR already addresses this by specifically explaining that these are only a convention, and any tool can basically use any output it wants.

I agree that this PR right now is not perfect, though. Every output should be linked to the tool that uses it, and the formatting needs to be fixed.

Apart from that, this seems like a very important addition to merge.