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

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 <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20200905204326.1378339-3-j.neuschaefer@gmx.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>