Documentation: filesystems: Convert ufs.txt to reStructuredText format

This converts the plain text documentation of ufs.txt to
reStructuredText format. Added to documentation build process
and verified with make htmldocs

Signed-off-by: Shobhit Kukreti <shobhitkukreti@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>