docs: ubifs-authentication: Add a top-level heading

This prevents the chapter headings from showing up in the table of
contents in filesystems/index.html.

Note that I didn't pick "UBIFS Authentication" as the document title,
because there is a chapter of the same name, and Sphinx complains about
multiple headings with the same name:

/.../Documentation/filesystems/ubifs-authentication.rst:207:
WARNING: duplicate label filesystems/ubifs-authentication:ubifs
authentication, other instance in
/.../Documentation/filesystems/ubifs-authentication.rst

Remove the :orphan: tag, as the document has been included into the
toctree.

Signed-off-by: Jonathan Neuschäfer <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/ubifs-authentication.rst

···
 .. SPDX-License-Identifier: GPL-2.0
 
-:orphan:
-
 .. UBIFS Authentication
 .. sigma star gmbh
 .. 2018
+
+============================
+UBIFS Authentication Support
+============================
 
 Introduction
 ============