docs: filesystems: convert zonefs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/42a7cfcd19f6b904a9a3188fd4af71bed5050052.1581955849.git.mchehab+huawei@kernel.org


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