nixos/manual: remove links from program listings

markdown cannot represent those links. remove them all now instead of in
each chapter conversion to keep the diff for each chapter small and more
understandable.

nixos/modules/programs/digitalbitbox/doc.xml

@@ -12,7 +12,7 @@
   setting <literal>programs.digitalbitbox</literal> to <literal>true</literal>
   in a manner similar to
 <programlisting>
-<xref linkend="opt-programs.digitalbitbox.enable"/> = true;
+programs.digitalbitbox.enable = true;
 </programlisting>
   and bundles the <literal>digitalbitbox</literal> package (see
   <xref
@@ -40,7 +40,7 @@
    <literal>digitalbitbox</literal> package which could be installed as
    follows:
 <programlisting>
-<xref linkend="opt-environment.systemPackages"/> = [
+environment.systemPackages = [
   pkgs.digitalbitbox
 ];
 </programlisting>
@@ -53,7 +53,7 @@
    The digitalbitbox hardware package enables the udev rules for Digital Bitbox
    devices and may be installed as follows:
 <programlisting>
-<xref linkend="opt-hardware.digitalbitbox.enable"/> = true;
+hardware.digitalbitbox.enable = true;
 </programlisting>
   </para>
 
@@ -63,8 +63,8 @@
    by means of overriding as follows:
 <programlisting>
 programs.digitalbitbox = {
-  <link linkend="opt-programs.digitalbitbox.enable">enable</link> = true;
-  <link linkend="opt-programs.digitalbitbox.package">package</link> = pkgs.digitalbitbox.override {
+  enable = true;
+  package = pkgs.digitalbitbox.override {
     udevRule51 = "something else";
   };
 };

nixos/modules/programs/plotinus.xml

@@ -24,7 +24,7 @@
   To enable Plotinus, add the following to your
   <filename>configuration.nix</filename>:
 <programlisting>
-<xref linkend="opt-programs.plotinus.enable"/> = true;
+programs.plotinus.enable = true;
 </programlisting>
  </para>
 </chapter>