docs: filesystems: convert affs.txt to ReST

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

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/b44c56befe0e28cbc0eb1b3e281ad7d99737ff16.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>