doc: Promote config Options Reference to sub-chapter section

It was hidden in the insecure packages section.
 - Lift it out.
 - Add an id

doc/using/configuration.chapter.md

@@ -164,14 +164,6 @@ There are several ways to tweak how Nix handles a package which has been marked
 
     Note that `permittedInsecurePackages` is only checked if `allowInsecurePredicate` is not specified.
 
-### `config` Options Reference
-
-The following attributes can be passed in [`config`](#chap-packageconfig).
-
-```{=docbook}
-<include xmlns="http://www.w3.org/2001/XInclude" href="../doc-support/result/config-options.docbook.xml"/>
-```
-
 ## Modify packages via `packageOverrides` {#sec-modify-via-packageOverrides}
 
 You can define a function called `packageOverrides` in your local `~/.config/nixpkgs/config.nix` to override Nix packages. It must be a function that takes pkgs as an argument and returns a modified set of packages.
@@ -184,6 +176,15 @@ You can define a function called `packageOverrides` in your local `~/.config/nix
 }
 ```
 
+## `config` Options Reference {#sec-config-options-reference}
+
+The following attributes can be passed in [`config`](#chap-packageconfig).
+
+```{=docbook}
+<include xmlns="http://www.w3.org/2001/XInclude" href="../doc-support/result/config-options.docbook.xml"/>
+```
+
+
 ## Declarative Package Management {#sec-declarative-package-management}
 
 ### Build an environment {#sec-building-environment}