docs: filesystems: convert afs.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Comment out text-only ToC;
- 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/d77f5afdb5da0f8b0ec3dbe720aef23f1ce73bb5.1581955849.git.mchehab+huawei@kernel.org


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