docs: filesystems: convert squashfs.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cec30862c7ee7de7f9cd903e35e6c8bf74cc928a.1581955849.git.mchehab+huawei@kernel.org


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