Add xfce documentation

Changed files
+1
nixos
doc
manual
configuration
+1
nixos/doc/manual/configuration/configuration.xml
···
<xi:include href="user-mgmt.xml" />
<xi:include href="file-systems.xml" />
<xi:include href="x-windows.xml" />
<xi:include href="networking.xml" />
<xi:include href="linux-kernel.xml" />
···
<xi:include href="user-mgmt.xml" />
<xi:include href="file-systems.xml" />
<xi:include href="x-windows.xml" />
+
<xi:include href="xfce.xml" />
<xi:include href="networking.xml" />
<xi:include href="linux-kernel.xml" />