doc: Show both the "bare-bones" and the "desktop" configurations.

* doc/guix.texi (System Installation): Add xref to "Using the
  Configuration System" instead of including one here.
  (Using the Configuration System): Remove first example, and include
  os-config-bare-bones.texi instead.  Include os-config-desktop.texi as
  a second example.
* doc.am (OS_CONFIG_EXAMPLES_TEXI): New variable.
  (BUILT_SOURCES, EXTRA_DIST, MAINTAINERCLEANFILES): Use it.
  (doc/os-config.texi): Remove.
  (doc/os-config-%.texi): New target.