Documentation: filesystems: Convert jfs.txt to

This converts the plain text documentation of jfs.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>