docs: filesystems: convert adfs.txt to ReST

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

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/15ee92f03ec917e5d26bd7b863565dec88c843f6.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>