doc: Add "lightweight desktop" OS config example.

* gnu/system/examples/desktop.tmpl (packages): Remove XFCE and
RATPOISON.
(services): Add 'gnome-desktop-service' and 'xfce-desktop-service'.
* gnu/system/examples/lightweight-desktop.tmpl: New file.
* Makefile.am (EXAMPLES): Add it.
* doc.am (OS_CONFIG_EXAMPLES_TEXI): Add
doc/os-config-lightweight-desktop.texi.
* gnu/system/install.scm (/etc/configuration-files)[directory]: Add
lightweight-desktop.tmpl.

gnu/system/examples/desktop.tmpl

@@ -1,9 +1,9 @@
 ;; This is an operating system configuration template
-;; for a "desktop" setup with X11.
+;; for a "desktop" setup with GNOME and Xfce.
 
 (use-modules (gnu) (gnu system nss))
 (use-service-modules desktop)
-(use-package-modules xfce ratpoison certs)
+(use-package-modules certs)
 
 (operating-system
   (host-name "antelope")
@@ -29,15 +29,17 @@
                 (home-directory "/home/bob"))
                %base-user-accounts))
 
-  ;; Add Xfce and Ratpoison; that allows us to choose
-  ;; sessions using either of these at the log-in screen.
-  (packages (cons* xfce ratpoison    ;desktop environments
-                   nss-certs         ;for HTTPS access
+  ;; This is where we specify system-wide packages.
+  (packages (cons* nss-certs         ;for HTTPS access
                    %base-packages))
 
-  ;; Use the "desktop" services, which include the X11
-  ;; log-in service, networking with Wicd, and more.
-  (services %desktop-services)
+  ;; Add GNOME and/or Xfce---we can choose at the log-in
+  ;; screen with F1.  Use the "desktop" services, which
+  ;; include the X11 log-in service, networking with Wicd,
+  ;; and more.
+  (services (cons* (gnome-desktop-service)
+                   (xfce-desktop-service)
+                   %desktop-services))
 
   ;; Allow resolution of '.local' host names with mDNS.
   (name-service-switch %mdns-host-lookup-nss))