config.nix: add optional docPrefix

Makes it possible to render it in the NixOS manual,
as that one does not contain the Nixpkgs manual specific ones.

pkgs/top-level/config.nix

@@ -6,9 +6,14 @@
 #     nix-build -A tests.config
 #
 
-{ config, lib, ... }:
+{
+  config,
+  lib,
+  ...
+}@args:
 
 let
+  docPrefix = args.docPrefix or "";
   inherit (lib)
     literalExpression
     mapAttrsToList
@@ -115,13 +120,13 @@ let
     gitConfig = mkOption {
       type = types.attrsOf (types.attrsOf types.anything);
       description = ''
-        The default [git configuration](https://git-scm.com/docs/git-config#_variables) for all [`pkgs.fetchgit`](#fetchgit) calls.
+        The default [git configuration](https://git-scm.com/docs/git-config#_variables) for all [`pkgs.fetchgit`](${docPrefix}#fetchgit) calls.
 
         Among many other potential uses, this can be used to override URLs to point to local mirrors.
 
         Changing this will not cause any rebuilds because `pkgs.fetchgit` produces a [fixed-output derivation](https://nix.dev/manual/nix/stable/glossary.html?highlight=fixed-output%20derivation#gloss-fixed-output-derivation).
 
-        To set the configuration file directly, use the [`gitConfigFile`](#opt-gitConfigFile) option instead.
+        To set the configuration file directly, use the [`gitConfigFile`](${docPrefix}#opt-gitConfigFile) option instead.
 
         To set the configuration file for individual calls, use `fetchgit { gitConfigFile = "..."; }`.
       '';
@@ -135,9 +140,9 @@ let
     gitConfigFile = mkOption {
       type = types.nullOr types.path;
       description = ''
-        A path to a [git configuration](https://git-scm.com/docs/git-config#_variables) file, to be used for all [`pkgs.fetchgit`](#fetchgit) calls.
+        A path to a [git configuration](https://git-scm.com/docs/git-config#_variables) file, to be used for all [`pkgs.fetchgit`](${docPrefix}#fetchgit) calls.
 
-        This overrides the [`gitConfig`](#opt-gitConfig) option, see its documentation for more details.
+        This overrides the [`gitConfig`](${docPrefix}#opt-gitConfig) option, see its documentation for more details.
       '';
       default =
         if config.gitConfig != { } then
@@ -155,7 +160,7 @@ let
 
         For example, an override like `"registry.npmjs.org" = "my-mirror.local/registry.npmjs.org"` will replace a URL like `https://registry.npmjs.org/foo.tar.gz` with `https://my-mirror.local/registry.npmjs.org/foo.tar.gz`.
 
-        To set the string directly, see [`npmRegistryOverridesString`](#opt-npmRegistryOverridesString).
+        To set the string directly, see [`npmRegistryOverridesString`](${docPrefix}#opt-npmRegistryOverridesString).
       '';
       default = { };
       example = {
@@ -174,7 +179,7 @@ let
       description = ''
         A string containing a string with a JSON representation of npm registry overrides for `fetchNpmDeps`.
 
-        This overrides the [`npmRegistryOverrides`](#opt-npmRegistryOverrides) option, see its documentation for more details.
+        This overrides the [`npmRegistryOverrides`](${docPrefix}#opt-npmRegistryOverrides) option, see its documentation for more details.
       '';
       default = builtins.toJSON config.npmRegistryOverrides;
     };
@@ -412,7 +417,7 @@ let
       type = types.listOf types.str;
       default = [ "https://tarballs.nixos.org" ];
       description = ''
-        The set of content-addressed/hashed mirror URLs used by [`pkgs.fetchurl`](#sec-pkgs-fetchers-fetchurl).
+        The set of content-addressed/hashed mirror URLs used by [`pkgs.fetchurl`](${docPrefix}#sec-pkgs-fetchers-fetchurl).
         In case `pkgs.fetchurl` can't download from the given URLs,
         it will try the hashed mirrors based on the expected output hash.