doc: Mention "audio" and "video" groups in configuration template.

* gnu/system/os-config.tmpl (users): Add "audio" and "video" groups and
  explain.

Merge branch 'master' into core-updates

install: Add a configuration template to the image.

* gnu/system/os-config.tmpl: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it
* gnu/system/install.scm (configuration-template-service): New
  procedure.
  (installation-services): Call it.
* doc/guix.texi (System Installation): Mention
  configuration-template.scm, and @include gnu/system/os-config.tmpl.