docs: filesystems: convert hfs.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Use notes markups;
- Add lists markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8a625d6652d88809730020048d26c3b9333ddbdf.1581955849.git.mchehab+huawei@kernel.org


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