docs: filesystems: split spufs.txt into 3 separate files

This file has manpages for 3 different things. As we'll
be converting it to ReST, let's fist split it into their
individual components.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3753aa73524f4e1cbf0c19e34f7b322420e0b1c6.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>