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>

Documentation/admin-guide/index.rst

@@ -72,6 +72,7 @@ configure specific aspects of kernel behavior to your liking.
    clearing-warn-once
    cpu-load
    cputopology
+   dell_rbu
    device-mapper/index
    efi-stub
    ext4