docs: filesystems: convert ubifs-authentication.rst.txt to ReST

- Add a SPDX header;
- Mark some literals as such;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0c36091b6660cd372f994bd98e1264491d766c22.1581955849.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>