docs: filesystems: add overlayfs to index.rst

While the document is there, it is currently missing at the
index file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3b8e7783b1fcc71e4f94af5ea8e5fa264392f8c4.1580193653.git.mchehab+huawei@kernel.org


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