docs: admin-guide: Move Dell RBU document from driver-api

This document describes how an admin can use the dell_rbu driver, rather
than any in-kernel API details.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>