doc: Rename OS config example.

* gnu/system/os-config.tmpl: Rename to...
* gnu/system/examples/bare-bones.tmpl: ... this.
* Makefile.am (EXAMPLES): Adjust accordingly.
* doc.am (doc/os-config.texi): Likewise.
* gnu/system/install.scm (configuration-template-service)[template]:
  Likewise.