docs: Move binderfs to admin-guide

The documentation is more appropriate for the administrator than for
the internal kernel API section it is currently in.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>