docs: Add remaining Documentation/*.txt at staging/index.rst

docs: Add remaining Documentation/*.txt at staging/index.rst

There are a few text files that aren't yet at the docs building
system.

Add those to the staging/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/35967d18de8b3ee480194acf68e62d515347c1e5.1592918949.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>